With #404 and #402 all routes now have the same types and thus we don't need to nest them but can instead store them all in a map. This simplifies the routing quite a bit and is faster as well.

High level changes:
- Routes are now stored in a `HashMap<RouteId, Route<B>>`.
- `Router::or` is renamed to `Router::merge` because thats what it does now. It copies all routes from one router to another. This also means overlapping routes will cause a panic which is nice win.
- `Router::merge` now only accepts `Router`s so added `Router::fallback` for adding a global 404 handler.
- The `Or` service has been removed.
- `Router::layer` now only adds layers to the routes you actually have meaning middleware runs _after_ routing. I believe that addresses #380 but will test that on another branch.

- Overall:
  - **fixed:** All known compile time issues are resolved, including those with
    `boxed` and those introduced by Rust 1.56 ([#404])
  - **breaking:** The router's type is now always `Router` regardless of how many routes or
    middleware are applied ([#404])

    This means router types are all always nameable:

    ```rust
    fn my_routes() -> Router {
        Router::new().route(
            "/users",
            post(|| async { "Hello, World!" }),
        )
    }
    ```
  - **breaking:** Added feature flags for HTTP1 and JSON. This enables removing a
    few dependencies if your app only uses HTTP2 or doesn't use JSON. Its only a
    breaking change if you depend on axum with `default_features = false`. ([#286])
  - **breaking:** `Route::boxed` and `BoxRoute` have been removed as they're no longer
    necessary ([#404])
  - **breaking:** `Nested`, `Or` types are now private. They no longer had to be
    public because `Router` is internally boxed ([#404])
  - **breaking:** Remove `routing::Layered` as it didn't actually do anything and
    thus wasn't necessary
  - **breaking:** Vendor `AddExtensionLayer` and `AddExtension` to reduce public
    dependencies
  - **breaking:** `body::BoxBody` is now a type alias for
    `http_body::combinators::UnsyncBoxBody` and thus is no longer `Sync`. This
    is because bodies are streams and requiring streams to be `Sync` is
    unnecessary.
  - **added:** Implement `IntoResponse` for `http_body::combinators::UnsyncBoxBody`.
  - **added:** Add `Handler::into_make_service` for serving a handler without a
    `Router`.
  - **added:** Add `Handler::into_make_service_with_connect_info` for serving a
    handler without a `Router`, and storing info about the incoming connection.
  - **breaking:** axum's minimum support rust version is not 1.54
- Routing:
  - Big internal refactoring of routing leading to several improvements ([#363])
    - **added:** Wildcard routes like `.route("/api/users/*rest", service)` are now supported.
    - **fixed:** The order routes are added in no longer matters.
    - **fixed:** Adding a conflicting route will now cause a panic instead of silently making
      a route unreachable.
    - **fixed:** Route matching is faster as number of routes increase.
  - **fixed:** Correctly handle trailing slashes in routes:
    - If a route with a trailing slash exists and a request without a trailing
      slash is received, axum will send a 301 redirection to the route with the
      trailing slash.
    - Or vice versa if a route without a trailing slash exists and a request
      with a trailing slash is received.
    - This can be overridden by explicitly defining two routes: One with and one
      without trailing a slash.
  - **breaking:** Method routing for handlers have been moved from `axum::handler`
    to `axum::routing`. So `axum::handler::get` now lives at `axum::routing::get`
    ([#405])
  - **breaking:** Method routing for services have been moved from `axum::service`
    to `axum::routing`. So `axum::service::get` now lives at, etc.
    `axum::routing::service_method_routing::get`, etc. ([#405])
  - **breaking:** `Router::or` renamed to `Router::merge` and will now panic on
    overlapping routes. It now only accepts `Router`s and not general `Service`s.
    Use `Router::fallback` for adding fallback routes ([#408])
  - **added:** `Router::fallback` for adding handlers for request that didn't
    match any routes. `Router::fallback` must be use instead of `nest("/", _)` ([#408])
  - **breaking:** `EmptyRouter` has been renamed to `MethodNotAllowed` as its only
    used in method routers and not in path routers (`Router`)
  - **breaking:** Remove support for routing based on the `CONNECT` method. An
    example of combining axum with and HTTP proxy can be found [here][proxy] ([#428])
- Extractors:
  - **fixed:** Expand accepted content types for JSON requests ([#378])
  - **fixed:** Support deserializing `i128` and `u128` in `extract::Path`
  - **breaking:** Automatically do percent decoding in `extract::Path`
    ([#272])
  - **breaking:** Change `Connected::connect_info` to return `Self` and remove
    the associated type `ConnectInfo` ([#396])
  - **added:** Add `extract::MatchedPath` for accessing path in router that
    matched the request ([#412])
- Error handling:
  - **breaking:** Simplify error handling model ([#402]):
    - All services part of the router are now required to be infallible.
    - Error handling utilities have been moved to an `error_handling` module.
    - `Router::check_infallible` has been removed since routers are always
      infallible with the error handling changes.
    - Error handling closures must now handle all errors and thus always return
      something that implements `IntoResponse`.

    With these changes handling errors from fallible middleware is done like so:

    ```rust,no_run
    use axum::{
        routing::get,
        http::StatusCode,
        error_handling::HandleErrorLayer,
        response::IntoResponse,
        Router, BoxError,
    };
    use tower::ServiceBuilder;
    use std::time::Duration;

    let middleware_stack = ServiceBuilder::new()
        // Handle errors from middleware
        //
        // This middleware most be added above any fallible
        // ones if you're using `ServiceBuilder`, due to how ordering works
        .layer(HandleErrorLayer::new(handle_error))
        // Return an error after 30 seconds
        .timeout(Duration::from_secs(30));

    let app = Router::new()
        .route("/", get(|| async { /* ... */ }))
        .layer(middleware_stack);

    fn handle_error(_error: BoxError) -> impl IntoResponse {
        StatusCode::REQUEST_TIMEOUT
    }
    ```

    And handling errors from fallible leaf services is done like so:

    ```rust
    use axum::{
        Router, service,
        body::Body,
        routing::service_method_routing::get,
        response::IntoResponse,
        http::{Request, Response},
        error_handling::HandleErrorExt, // for `.handle_error`
    };
    use std::{io, convert::Infallible};
    use tower::service_fn;

    let app = Router::new()
        .route(
            "/",
            get(service_fn(|_req: Request<Body>| async {
                let contents = tokio::fs::read_to_string("some_file").await?;
                Ok::<_, io::Error>(Response::new(Body::from(contents)))
            }))
            .handle_error(handle_io_error),
        );

    fn handle_io_error(error: io::Error) -> impl IntoResponse {
        // ...
    }
    ```
- Misc:
  - `InvalidWebsocketVersionHeader` has been renamed to `InvalidWebSocketVersionHeader` ([#416])
  - `WebsocketKeyHeaderMissing` has been renamed to `WebSocketKeyHeaderMissing` ([#416])

[#339]: #339
[#286]: #286
[#272]: #272
[#378]: #378
[#363]: #363
[#396]: #396
[#402]: #402
[#404]: #404
[#405]: #405
[#408]: #408
[#412]: #412
[#416]: #416
[#428]: #428
[proxy]: https://github.com/tokio-rs/axum/blob/main/examples/http-proxy/src/main.rs

- Overall:
  - **fixed:** All known compile time issues are resolved, including those with
    `boxed` and those introduced by Rust 1.56 ([#404])
  - **breaking:** The router's type is now always `Router` regardless of how many routes or
    middleware are applied ([#404])

    This means router types are all always nameable:

    ```rust
    fn my_routes() -> Router {
        Router::new().route(
            "/users",
            post(|| async { "Hello, World!" }),
        )
    }
    ```
  - **breaking:** Added feature flags for HTTP1 and JSON. This enables removing a
    few dependencies if your app only uses HTTP2 or doesn't use JSON. Its only a
    breaking change if you depend on axum with `default_features = false`. ([#286])
  - **breaking:** `Route::boxed` and `BoxRoute` have been removed as they're no longer
    necessary ([#404])
  - **breaking:** `Nested`, `Or` types are now private. They no longer had to be
    public because `Router` is internally boxed ([#404])
  - **breaking:** Remove `routing::Layered` as it didn't actually do anything and
    thus wasn't necessary
  - **breaking:** Vendor `AddExtensionLayer` and `AddExtension` to reduce public
    dependencies
  - **breaking:** `body::BoxBody` is now a type alias for
    `http_body::combinators::UnsyncBoxBody` and thus is no longer `Sync`. This
    is because bodies are streams and requiring streams to be `Sync` is
    unnecessary.
  - **added:** Implement `IntoResponse` for `http_body::combinators::UnsyncBoxBody`.
  - **added:** Add `Handler::into_make_service` for serving a handler without a
    `Router`.
  - **added:** Add `Handler::into_make_service_with_connect_info` for serving a
    handler without a `Router`, and storing info about the incoming connection.
  - **breaking:** axum's minimum support rust version is not 1.54
- Routing:
  - Big internal refactoring of routing leading to several improvements ([#363])
    - **added:** Wildcard routes like `.route("/api/users/*rest", service)` are now supported.
    - **fixed:** The order routes are added in no longer matters.
    - **fixed:** Adding a conflicting route will now cause a panic instead of silently making
      a route unreachable.
    - **fixed:** Route matching is faster as number of routes increase.
  - **fixed:** Correctly handle trailing slashes in routes:
    - If a route with a trailing slash exists and a request without a trailing
      slash is received, axum will send a 301 redirection to the route with the
      trailing slash.
    - Or vice versa if a route without a trailing slash exists and a request
      with a trailing slash is received.
    - This can be overridden by explicitly defining two routes: One with and one
      without trailing a slash.
  - **breaking:** Method routing for handlers have been moved from `axum::handler`
    to `axum::routing`. So `axum::handler::get` now lives at `axum::routing::get`
    ([#405])
  - **breaking:** Method routing for services have been moved from `axum::service`
    to `axum::routing`. So `axum::service::get` now lives at, etc.
    `axum::routing::service_method_routing::get`, etc. ([#405])
  - **breaking:** `Router::or` renamed to `Router::merge` and will now panic on
    overlapping routes. It now only accepts `Router`s and not general `Service`s.
    Use `Router::fallback` for adding fallback routes ([#408])
  - **added:** `Router::fallback` for adding handlers for request that didn't
    match any routes. `Router::fallback` must be use instead of `nest("/", _)` ([#408])
  - **breaking:** `EmptyRouter` has been renamed to `MethodNotAllowed` as its only
    used in method routers and not in path routers (`Router`)
  - **breaking:** Remove support for routing based on the `CONNECT` method. An
    example of combining axum with and HTTP proxy can be found [here][proxy] ([#428])
- Extractors:
  - **fixed:** Expand accepted content types for JSON requests ([#378])
  - **fixed:** Support deserializing `i128` and `u128` in `extract::Path`
  - **breaking:** Automatically do percent decoding in `extract::Path`
    ([#272])
  - **breaking:** Change `Connected::connect_info` to return `Self` and remove
    the associated type `ConnectInfo` ([#396])
  - **added:** Add `extract::MatchedPath` for accessing path in router that
    matched the request ([#412])
- Error handling:
  - **breaking:** Simplify error handling model ([#402]):
    - All services part of the router are now required to be infallible.
    - Error handling utilities have been moved to an `error_handling` module.
    - `Router::check_infallible` has been removed since routers are always
      infallible with the error handling changes.
    - Error handling closures must now handle all errors and thus always return
      something that implements `IntoResponse`.

    With these changes handling errors from fallible middleware is done like so:

    ```rust,no_run
    use axum::{
        routing::get,
        http::StatusCode,
        error_handling::HandleErrorLayer,
        response::IntoResponse,
        Router, BoxError,
    };
    use tower::ServiceBuilder;
    use std::time::Duration;

    let middleware_stack = ServiceBuilder::new()
        // Handle errors from middleware
        //
        // This middleware most be added above any fallible
        // ones if you're using `ServiceBuilder`, due to how ordering works
        .layer(HandleErrorLayer::new(handle_error))
        // Return an error after 30 seconds
        .timeout(Duration::from_secs(30));

    let app = Router::new()
        .route("/", get(|| async { /* ... */ }))
        .layer(middleware_stack);

    fn handle_error(_error: BoxError) -> impl IntoResponse {
        StatusCode::REQUEST_TIMEOUT
    }
    ```

    And handling errors from fallible leaf services is done like so:

    ```rust
    use axum::{
        Router, service,
        body::Body,
        routing::service_method_routing::get,
        response::IntoResponse,
        http::{Request, Response},
        error_handling::HandleErrorExt, // for `.handle_error`
    };
    use std::{io, convert::Infallible};
    use tower::service_fn;

    let app = Router::new()
        .route(
            "/",
            get(service_fn(|_req: Request<Body>| async {
                let contents = tokio::fs::read_to_string("some_file").await?;
                Ok::<_, io::Error>(Response::new(Body::from(contents)))
            }))
            .handle_error(handle_io_error),
        );

    fn handle_io_error(error: io::Error) -> impl IntoResponse {
        // ...
    }
    ```
- Misc:
  - `InvalidWebsocketVersionHeader` has been renamed to `InvalidWebSocketVersionHeader` ([#416])
  - `WebsocketKeyHeaderMissing` has been renamed to `WebSocketKeyHeaderMissing` ([#416])

[#339]: #339
[#286]: #286
[#272]: #272
[#378]: #378
[#363]: #363
[#396]: #396
[#402]: #402
[#404]: #404
[#405]: #405
[#408]: #408
[#412]: #412
[#416]: #416
[#428]: #428
[proxy]: https://github.com/tokio-rs/axum/blob/main/examples/http-proxy/src/main.rs