Store richer adapter types, don't use instructions for TypeScript

[alexcrichton]

This commit updates how TypeScript signature are generated from adapters
in wasm-bindgen. A richer set of AdapterType types are now stored
which record information about optional types and such. These direct
AdapterType values are then used to calculate the TypeScript
signature, rather than following the instructions in an adapter function
(which only works anyway for wasm-bindgen generated adapters).

This should be more robust since it reads the actual true signature of
the adapter to generate the TypeScript signature, rather than attempting
to ad-hoc-ly infer it from the various instructions, which was already
broken.

A number of refactorings were involved here, but the main pieces are:

• The AdapterType type is a bit more rich now to describe more
  Rust-like types.
• The TypescriptArg structure is now gone and instead return values
  are directly inferred from type signatures of adapters.
• The typescript_{required,optional} methods are no longer needed.
• The return of JsBuilder::process was enhanced to return more values,
  rather than storing some return values on the structure itself.

Closes #1926