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 is easier to access as ec->ractor_id instead of pointer-chasing through
ec->thread->ractor->ractor_id

Co-authored-by: Luke Gruber <luke.gru@gmail.com>

Before this change, GC'ing any Ractor object caused you to lose all
enabled tracepoints across all ractors (even main). Now tracepoints are
ractor-local and this doesn't happen. Internal events are still global.

Fixes [Bug #19112]

When defining a bmethod, we recorded the current Ractor's object in the
method. However that was never marked and so could be GC'd and reused by
a future Ractor. Instead we can use the Ractor's id, which we expect to
be unique forever.

Co-authored-by: Luke Gruber <luke.gru@gmail.com>

Recently rb_vm_getinstancevariable was introduced exposing this method
to ZJIT. On clang specifically this ended up causing the compiler not to
inline into vm_exec_core and cause a significant performance regression
in optcarrot for the interpreter.

Co-authored-by: Luke Gruber <luke.gru@gmail.com>

[Bug #21694]

`vm_search_super_method` explictly calls `vm_cc_new` with `cme=NULL`
when there is no super class.

As can be seen in vm_block_handler_verify(), VM_BLOCK_HANDLER_NONE is
not a valid argument for vm_block_handler(). Store nil in the profiler
when seen instead of crashing.