undefine alloc functions for C extensions

	per guidance in doc/extension.rdoc, these classes now undefine their
	alloc functions:

	- ObjectSpace::InternalObjectWrapper
	- Socket::Ifaddr
	---
	 ext/objspace/objspace.c | 1 +
	 ext/socket/ifaddr.c     | 1 +
	 2 files changed, 2 insertions(+)

constant cache `IC` is accessed by non-atomic manner and there are
thread-safety issues, so Ruby 3.0 disables to use const cache on
non-main ractors.

This patch enables it by introducing `imemo_constcache` and allocates
it by every re-fill of const cache like `imemo_callcache`.
[Bug #17510]

Now `IC` only has one entry `IC::entry` and it points to
`iseq_inline_constant_cache_entry`, managed by T_IMEMO object.

`IC` is atomic data structure so `rb_mjit_before_vm_ic_update()` and
`rb_mjit_after_vm_ic_update()` is not needed.

This iterator uses an st_table, but if objects move the references in
the st table won't be updated.  This patch just changes the st table to
an identity hash.

We have code common to all heap iteration paths in this file.  Refactor
such that we keep ASAN checks and flags checks in one place

Again, this code is walking the heap.  Empty slots can be poisoned, so
we need to unpoison before checking the type

ext/objspace iterates over the heap, but some slots in the heap are
poisoned, so we need to take care of that when running with ASAN

The added test case crashes the interpreter because it makes
ObjectSpace.internal_class_of return the second VALUE slot of an AST
imemo object. The second VALUE slot of `struct rb_ast_struct` is
not a VALUE and not a pointer to a Ruby object.

It seems almost no internal codes use RHASH_TBL any longer.  Why not
just eliminate it entirely, so that the macro can be purely ext-only.