This will allow the Ruby backtrace, memory mapping, etc. to be outputted
when a Ruby mutator thread panics.

https://github.com/ruby/mmtk/commit/d10fd325dd

If a reference marked weak becomes a special const, it will crash because
it is not a GC handled object. We should skip special consts here.

https://github.com/ruby/mmtk/commit/870a79426b

We also enable `#![warn(unsafe_op_in_unsafe_fn)]` in the whole mmtk_ruby
crate.

https://github.com/ruby/mmtk/commit/8b8025f71a

Remove the unused constant HAS_MOVED_GFIELDSTBL and related methods.

In the mmtk/mmtk-ruby repo, we are now able to find the global field
(IV) table of a moved object during copying GC without using the
HAS_MOVED_GFIELDSTBL bit.  We synchronize some of the code, although we
haven't implemented moving GC in ruby/mmtk, yet.

See: https://github.com/mmtk/mmtk-ruby/commit/13080acdf553f20a88a7ea9ab9f6877611017136

https://github.com/ruby/mmtk/commit/400ba4e747

And get rid of the `obj_to_id_tbl`

It's no longer needed, the `object_id` is now stored inline
in the object alongside instance variables.

We still need the inverse table in case `_id2ref` is invoked, but
we lazily build it by walking the heap if that happens.

The `object_id` concern is also no longer a GC implementation
concern, but a generic implementation.

Co-Authored-By: Matt Valentine-House <matt@eightbitraptor.com>

Ivars will longer be the only thing stored inline
via shapes, so keeping the `iv_index` and `ivptr` names
would be confusing.

Instance variables won't be the only thing stored inline
via shapes, so keeping the `ivptr` name would be confusing.

`field` encompass anything that can be stored in a VALUE array.

Similarly, `gen_ivtbl` becomes `gen_fields_tbl`.

We rely on scan_vm_specific_roots to reach all stacks via the following
path:

    VM -> ractors -> threads -> fibers -> stacks

https://github.com/ruby/mmtk/commit/0a6a835aaa

https://github.com/ruby/mmtk/commit/9da566e26a

This commit implements the mark-sweep algorithm using MMTk and allows
customizing the plan using MMTK_PLAN.

https://github.com/ruby/mmtk/commit/6fea5e5ffc

Co-Authored-By: Matt Valentine-House <matt@eightbitraptor.com>