[GC][EH] Fuzz exnref in GC structs

[kripken]

Most of the work here is avoiding shared and non-nullable exnref in
various places, as we cannot support those in the fuzzer (I don't see
a way to generate shared exnrefs, and non-nullable ones can be
created but not in global places, as the way to create them needs a
block and a try).

Some example output:

 (type $0 (struct (field (mut exnref)) (field (mut i64)) (field (mut v128))))
 (type $1 (sub (struct (field (ref null $1)) (field (mut i16)) (field (mut exnref)) (field (ref struct)))))
 (type $3 (sub final $1 (struct (field (ref null $3)) (field (mut i16)) (field (mut exnref)) (field (ref $2)))))

[aheejin]

LGTM % nits

[aheejin]

Given that we check for features.hasExceptionHandling() for exn, don't we also need to check for features.hasGC() or something for these?

[kripken]

I think this entire file is for fuzzing of heap types, so GC is assumed everywhere. This is a separate file from the main fuzzer.