It has been marked as obsolete for a while and I see no reason
to keep it.

This behave almost exactly as a T_OBJECT, the layout is entirely
compatible.

This aims to solve two problems.

First, it solves the problem of namspaced classes having
a single `shape_id`. Now each namespaced classext
has an object that can hold the namespace specific
shape.

Second, it open the door to later make class instance variable
writes atomics, hence be able to read class variables
without locking the VM.
In the future, in multi-ractor mode, we can do the write
on a copy of the `fields_obj` and then atomically swap it.

Considerations:

  - Right now the `RClass` shape_id is always synchronized,
    but with namespace we should likely mark classes that have
    multiple namespace with a specific shape flag.

This commit splits gc.c into two files:

- gc.c now only contains code not specific to Ruby GC. This includes
  code to mark objects (which the GC implementation may choose not to
  use) and wrappers for internal APIs that the implementation may need
  to use (e.g. locking the VM).

- gc_impl.c now contains the implementation of Ruby's GC. This includes
  marking, sweeping, compaction, and statistics. Most importantly,
  gc_impl.c only uses public APIs in Ruby and a limited set of functions
  exposed in gc.c. This allows us to build gc_impl.c independently of
  Ruby and plug Ruby's GC into itself.

Previous documentation is stating the opposite (that the method won't
work for CRuby).

Using RHASH_TBL_RAW is a private API, so we should use rb_hash_foreach
rather than RHASH_TBL_RAW with st_foreach.