[threads] Parse, build, and print shared composite types (#6654)

Parse the text format for shared composite types as described in the
shared-everything thread proposal. Update the parser to use 'comptype' instead
of 'strtype' to match the final GC spec and add the new syntactic class
'sharecomptype'.

Update the type canonicalization logic to take sharedness into account to avoid
merging shared and unshared types. Make the same change in the TypeMerging pass.
Ensure that shared and unshared types cannot be in a subtype relationship with
each other.

Follow-up PRs will add shared abstract heap types, binary parsing and emitting
for shared types, and fuzzer support for shared types.

0 files changed, 0 insertions, 0 deletions