Wasm: Environment interface

Expose a function rewrite : tree -> tree Lwt.t that performs a single tree rewrite, for use by PVM.eval.

The rewrite must call into tickified parsing/evaluation exposed in lib_webassembly. It should make use of lazy tree parsing/updating added in #3080 (closed) #3081 (closed) #3082 (closed) #3083 (closed).

On the Env side eval must dispatch to Decode.step Eval.init, and Eval.step depending on the machine state.

PVM.set_input is out of scope as it will need to be handled on the Protocol side.