class.c: rb_class_duplicate_classext also dup content of cvc_tbl

	[Bug #21952]

	Shallow copying the table result in the same memory being shared
	between multiple box, causing double free when one of the box
	is garbage collected.

	---

	class.c: Make cvc_tbl a managed object

	[Bug #21952]

	Solves the double-free or use after-free concern with boxes.
	Now entries can safely be used for copy-on-write.

	Also is likely necessary to make it save to read cvar from
	secondary ractors, as allowed since: ab32c0e690b805cdaaf264ad4c3421696c588204

This allows these methods to be called from ractors.

Add new exported function `rb_vm_lock_with_barrier()` that requires
users to include "vm_sync.h"

allocation_info_tracer_compact_update_object_table_i deletes entries where
the key is no longer in the GC heap but did not free the allocation_info
causing the memory to be leaked.

The embed layout is way more common than the heap one,
especially since WVA.

I think it makes for more readable code to inverse the
flag.

`RSHAPE_PARENT` is error prone because it returns a raw untagged
shape_id.

To check if a shape is a direct parent of another, tags should be
discarded. So providing a comparison function is better than exposing
untagged ids.

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