Refactor: Improve Set GC marking and freeing

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.

include/mruby/internal.h

@@ -133,6 +133,10 @@ mrb_value mrb_as_rational(mrb_state *mrb, mrb_value x);
 void mrb_rational_copy(mrb_state *mrb, mrb_value x, mrb_value y);
 int mrb_rational_mark(mrb_state *mrb, struct RBasic *rat);
 #endif
+#ifdef MRB_USE_SET
+size_t mrb_gc_mark_set(mrb_state *mrb, struct RBasic *set);
+void mrb_gc_free_set(mrb_state *mrb, struct RBasic *set);
+#endif
 
 #ifdef MRUBY_PROC_H
 struct RProc *mrb_closure_new(mrb_state*, const mrb_irep*);

include/mruby/value.h

@@ -163,7 +163,8 @@ static const unsigned int IEEE754_INFINITY_BITS_SINGLE = 0x7F800000;
   f(MRB_TT_COMPLEX,     struct RComplex,    "Complex") \
   f(MRB_TT_RATIONAL,    struct RRational,   "Rational") \
   f(MRB_TT_BIGINT,      struct RBigint,     "Integer") \
-  f(MRB_TT_BACKTRACE,   struct RBacktrace,  "backtrace")
+  f(MRB_TT_BACKTRACE,   struct RBacktrace,  "backtrace") \
+  f(MRB_TT_SET,         struct RData,       "Set")
 
 enum mrb_vtype {
 #define MRB_VTYPE_DEFINE(tt, type, name) tt,