Backend refactor: Move ActorScript name environment further down

[nomeata]

compile.ml is mostly split into two parts:

• the back-end leaning parts: Heap objects, functions on them etc.
• the frontend-leaning parts: Traversing the AST, keeping track of the
  variable bindings etc.

But so far we had one environment for both aspects (e.g. storing names
for WebAssembly locals; keeping track of where AS variables are stored).
This has bothered me for a long time, and I finally pulled the band-aid:

• E.t no longer has local_vars_env. In fact, E.t is only concerned
  with the bookkeeping needed to prodcue Wasm.

  Also env : E.t now only changes when going into a new Wasm function,
  no longer with every ActorScript scope. For some local go function,
  it no longer needs to be passed around.

• There is a new environment ASEnv.t which only contains the
  ActorScript name and label environment. It is defined much futher
  down, and passed around separately by compile_exp etc.

  Only the ae : ASEnv.t changes when entering a scope, so only this
  needs to be threaded through patterns or declarations etc.

This also unbreaks the unholy nested type recursion through varloc. I
expect that I can simply the deferred_loc story now that this is
distangeled.

In theory we should be able to do open IR before the second part, but
we are not quite there yet.

Bitwise identical Wasm output with this change, according to
./compare-wat.

[nomeata]

Gabor, can I have a review? This is a refactoring, so rather tedious to keep in a feature branch.

[ggreif]

Looks reasonable, let's see how it affects in-flight PRs and branches.