Ignore all annotations but entrypoints

Copied from the archived Metastate repo: metastatedev/tezos#113.

Annotations bring a complexity to the parser and the typechecker as well as unsatisfactory properties such as non-transitivity of type equality. They are useful when programming and debugging Michelson programs, but don't bring many strong guarantees, and aren't necessary once checked. Now that we have high-level languages targeting Michelson, annotations don't seem very useful to keep in the language.

Shall we just remove them?

If yes, we'll have to have discussions on:

• entrypoints
• migration & backward-compatibility

Steps:

• tzip: tzip!138 (closed)

1. Remove stack variable annotations: !4139 (merged)
2. Remove pair variable annotations: !4140 (merged)
3. Remove type annotations: !4141 (merged)
4. Remove field annotations from comparable types and pairs: !4175 (merged)
5. Remove field annotations from unions + refactor entrypoints:
   • !4311 (merged)
   • !4259 (merged)

• Update documentation
• Tests

6. Simplify merge functions:
   • !4297 (merged)
   • !4298 (merged)
7. Simplify checking annotations