Changes:
- Implement function subtyping in wasm-subtyping.cc.
- Add Signature::Build(), which takes initializer lists for the return
  and parameter types.
- Only throw kTrapFuncSigMismatch in call_indirect, change that trap's
  message.
- Add a missing "return 0" in function-body-decoder-impl.h
- Fix a faulty check in wasm-objects.cc.
- Improve some comments.
- Write tests. Improve readability of subtyping-unittest.

Bug: v8:7748
Change-Id: I1caba09d5bd01cfd4d6125f300cd9c16af7aba99
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2822633Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73972}