Work towards adding heap-allocated object support for wasm, according to
the gc proposal.

Changes:
- Implement subtyping for reference types (ref s) and (optref s),
  where 's' is a struct type.
  This CL does *not* implement subtyping between struct and function
  types. Also, it does not handle i31refs and eqrefs.
- Implement struct.set.
- Change struct.get to accept an optref as argument, as required by the
  standard.
- Allow locals to store objects of ref and optref types.
- Add a test for struct.set and optref locals. Modify the test for
  struct.get accordingly.

Reference: https://github.com/WebAssembly/gc

R=jkummerow@chromium.org
R=clemensb@chromium.org

Bug: v8:7748
Change-Id: I708626fa5f90a6e24e667d66eed1c7697f458a23
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2172089Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67562}