[PATCH] [Bug #21819] Data objects without members should also be frozen

	[Bug #21440] Stop caching member list in frozen Data/Struct class

Previously it would bypass the `FL_ABLE` check, but
since shapes introduction, it started having a different
behavior than `OBJ_FREEZE`, as it would onyl set the `FL_FREEZE`
flag, but not update the shape.

I have no indication of this causing a bug yet, but it seems
like a trap waiting to happen.

This `st_table` is used to both mark and pin classes
defined from the C API. But `vm->mark_object_ary` already
does both much more efficiently.

Currently a Ruby process starts with 252 rooted classes,
which uses `7224B` in an `st_table` or `2016B` in an `RArray`.

So a baseline of 5kB saved, but since `mark_object_ary` is
preallocated with `1024` slots but only use `405` of them,
it's a net `7kB` save.

`vm->mark_object_ary` is also being refactored.

Prior to this changes, `mark_object_ary` was a regular `RArray`, but
since this allows for references to be moved, it was marked a second
time from `rb_vm_mark()` to pin these objects.

This has the detrimental effect of marking these references on every
minors even though it's a mostly append only list.

But using a custom TypedData we can save from having to mark
all the references on minor GC runs.

Addtionally, immediate values are now ignored and not appended
to `vm->mark_object_ary` as it's just wasted space.

[Bug #20311]

`rb_define_class_under` assumes it's called from C and that the
reference might be held in a C global variable, so it adds the
class to the VM root.

In the case of `Struct.new('Name')` it's wasteful and make
the struct immortal.

[Feature #16495]