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.

xfree can handle null values, so we don't need to check it.

[Misc #18891]

rb_class_get_superclass returns the immediate SUPER, including T_ICLASS.
rb_ivar_lookup isn't implemented for T_ICLASS so it uses the default
behaviour, which always returns Qnil.

This commit avoids checking T_ICLASS for ivars.

* memory_view.c: Rename private to private_data for C++

* doc/memory_view.md: Update document

* Fix doc/memory_view.md

Co-authored-by: Sutou Kouhei <kou@cozmixng.org>

Co-authored-by: Sutou Kouhei <kou@cozmixng.org>

[Feature #18051]

* --braces-after-func-def-line
* --dont-cuddle-else
* --procnames-start-lines
* --space-after-for
* --space-after-if
* --space-after-while

* See [Feature #17752]

Co-authored-by: xtkoba (Tee KOBAYASHI) <xtkoba+ruby@gmail.com>