This commit addresses feedback on the initial Set GC marking implementation.
Changes include:
- Renamed set marking function to `mrb_gc_mark_set` and updated its
  return type to `size_t`.
- Introduced an explicit `mrb_gc_free_set` function for Set objects.
- Updated `gc_mark_children` to use the new mark function signature.
- Added an explicit `case MRB_TT_SET:` in `obj_free` to call `mrb_gc_free_set`.
- Adjusted `set_get_khash` in `mruby-set` to work with `MRB_TT_SET` directly,
  rather than relying on `mrb_data_get_ptr`.
- Corrected type checks in `set_init_copy` to use `MRB_TT_SET`.
- Updated function prototypes in internal headers and stubs in mrbc.