This CL replaces the equirecursive type system for wasm-gc with the
isorecursive hybrid type system presented here:
https://github.com/WebAssembly/gc/issues/257.

In broad strokes, this includes the following changes:
- In the module decoder, remove equirecursive types. Implement recursive
  type groups, subtype definitions, and function/struct/array
  definitions. Treat nominal modules as syntactic sugar of an
  isorecursive module, where all types belong in the same recursive
  group.
- Remove rtt.sub and all related infrastructure.
- Change subtyping to work with explicit supertypes only.
- Add ValidSubtypeDefinition in subtyping, to check that subtype
  declarations are valid during decoding.
- Remove the subtyping cache.
- Add support for functions to have specific signature index in
  WasmModuleBuilder and in test-gc.cc.
- Adapt tests.

Current restrictions:
- Recursive groups are not stored beyond decoding.
- Type canonicalization is not implemented. No tests relying on types
  being considered identical post-canonicalization.
- No cross-module subtyping is possible. Tests relying on cross-module
  subtyping have been commented out.

Bug: v8:7748
Change-Id: I69fd04ecc5611f6230c95d5c89d1c520163fffae
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3416239Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78816}