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.