Changes:
Cleanup:
- Make sure read_value_type has the same interface as other
  read_* functions, i.e., returns the decoded value and writes
  the consumed length into a pointer.
- DecodeLocals is now an instance method.
- DecodeLocals should fail when given a wrong number of locals.
  Add tests to catch that.
- Fix a buggy test.

Refactoring in preparation of introducing the 'let'
instruction as per [wasm-gc]:
- DecodeLocals does not consume any input and can start from any pc.
- DecodeLocals gives the option of not appending the decoded
  locals to local_types_.
- Separate locals initialization from signature.

Bug: v8:7748
Change-Id: Iaaff87fdb9abe0ddd716484ea3fa87779d2d1a2f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2202992
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67845}