gc_ops: Add support for struct subtypes (#12931)

* Add struct subtypes

* Supertypes with probability

Clean up `gc_ops` fuzzing mutator (#12533)

* Clean up `gc_ops` fuzzing mutator

This commit cleans up a few random things about the `gc_ops` fuzzing mutator,
notably:

* The macro to define `GcOp` a

Clean up `gc_ops` fuzzing mutator (#12533)

* Clean up `gc_ops` fuzzing mutator

This commit cleans up a few random things about the `gc_ops` fuzzing mutator,
notably:

* The macro to define `GcOp` and many of its methods is refactored to be more
extensible going forward (e.g. all of GC op `fixup` is now macro-generated,
instead of having special-cases for typed struct ops, and we could probably
also move encoding into the macro as a follow up PR).
* Furthermore, the macro is also a `for_each_*`-style macro now, which lets us
define the GC ops once and then use the macro multiple times to define
different things, rather than being forced to put everything we want to
generate into one macro RHS. This should make things a bit easier to read and
edit and plays nicer with LSP.
* A bunch of GC op filtering that was logically part of fixing up an instruction
is now actually part of `GcOp::fixup`. Also a redundant `op.fixup()` call was
removed, `GcOp::fixup` takes `&self` instead of `&mut self` since it returns a
new `GcOp` now instead of mutatint `self` in place, etc...
* A few other small things here and there.

* Fix `GcOps` fuzz mutator tests

Also remove the `test_wat_string` test because it isn't really helpful at this
point, it is just something we have to remember to update whenever we
add/remove/change `GcOp`s while not giving us anything in return.

show more ...

Struct locals etc (#12513)

* Add structref local/global/table access for structs

* The number of iters back to 2048

* Remove forgotten struct_num_local from limits

* Address initial reviews

* Id

Struct locals etc (#12513)

* Add structref local/global/table access for structs

* The number of iters back to 2048

* Remove forgotten struct_num_local from limits

* Address initial reviews

* Idiomatic comments

show more ...

Stack types (#12015)

* Stack fixup for new types

* Add new test for fixup()

* Address clippy failure

* Update fixup and num_types

* Remove vector allocation each time

* Address GcOps::generate

Stack types (#12015)

* Stack fixup for new types

* Add new test for fixup()

* Address clippy failure

* Update fixup and num_types

* Remove vector allocation each time

* Address GcOps::generate

* Remove println from tests

* Address the Anything stack type

* Address the second round of reviews

show more ...

Split modules (#11993)

* Split GC to different modules and rename table_ops to gc_ops

* Remove table_ops from fuzz_targets. Renamed to gc_ops

* Address formatting failures