Fix potentially use of wrong method cache

#### Example (with `MRB_METHOD_CACHE`)

  ```ruby
  GC.start
  c = Class.new
  p c            #=> #<Class:0x7fd6a180e790>
  c.new          #=> cache `c.new`
  c = nil
  GC.start       #=> `c` is GCed
  r = Range.dup
  p r            #=> #<Class:0x7fd6a180e790>
                 #   [same pointer as `c`]
  r.new(2, 3)    #=> ArgumentError: 'initialize':
                 #   wrong number of arguments (2 for 0)
                 #   [`c.new` is called instead of `r.new`]
  ```

#### Cause

  An entry of method cache is identified by class pointer and method
  id. However, reusing memory after GC may create a class with the same
  pointer as the cached class.

#### Treatment

  Cleared method caches of the class when the class is GCed.

include/mruby/class.h

@@ -95,6 +95,12 @@ void mrb_gc_mark_mt(mrb_state*, struct RClass*);
 size_t mrb_gc_mark_mt_size(mrb_state*, struct RClass*);
 void mrb_gc_free_mt(mrb_state*, struct RClass*);
 
+#ifdef MRB_METHOD_CACHE
+void mrb_mc_clear_by_class(mrb_state *mrb, struct RClass* c);
+#else
+#define mrb_mc_clear_by_class(mrb,c)
+#endif
+
 MRB_END_DECL
 
 #endif  /* MRUBY_CLASS_H */

src/class.c

@@ -285,11 +285,9 @@ mrb_define_class(mrb_state *mrb, const char *name, struct RClass *super)
 static mrb_value mrb_bob_init(mrb_state *mrb, mrb_value);
 #ifdef MRB_METHOD_CACHE
 static void mc_clear_all(mrb_state *mrb);
-static void mc_clear_by_class(mrb_state *mrb, struct RClass*);
 static void mc_clear_by_id(mrb_state *mrb, struct RClass*, mrb_sym);
 #else
 #define mc_clear_all(mrb)
-#define mc_clear_by_class(mrb,c)
 #define mc_clear_by_id(mrb,c,s)
 #endif
 
@@ -303,7 +301,7 @@ mrb_class_inherited(mrb_state *mrb, struct RClass *super, struct RClass *klass)
     super = mrb->object_class;
   super->flags |= MRB_FL_CLASS_IS_INHERITED;
   s = mrb_obj_value(super);
-  mc_clear_by_class(mrb, klass);
+  mrb_mc_clear_by_class(mrb, klass);
   mid = mrb_intern_lit(mrb, "inherited");
   if (!mrb_func_basic_p(mrb, s, mid, mrb_bob_init)) {
     mrb_value c = mrb_obj_value(klass);
@@ -1109,7 +1107,7 @@ include_module_at(mrb_state *mrb, struct RClass *c, struct RClass *ins_pos, stru
     m->flags |= MRB_FL_CLASS_IS_INHERITED;
     ins_pos->super = ic;
     mrb_field_write_barrier(mrb, (struct RBasic*)ins_pos, (struct RBasic*)ic);
-    mc_clear_by_class(mrb, ins_pos);
+    mrb_mc_clear_by_class(mrb, ins_pos);
     ins_pos = ic;
   skip:
     m = m->super;
@@ -1322,8 +1320,8 @@ mc_clear_all(mrb_state *mrb)
   }
 }
 
-static void
-mc_clear_by_class(mrb_state *mrb, struct RClass *c)
+void
+mrb_mc_clear_by_class(mrb_state *mrb, struct RClass *c)
 {
   struct mrb_cache_entry *mc = mrb->cache;
   int i;

src/gc.c

@@ -807,10 +807,12 @@ obj_free(mrb_state *mrb, struct RBasic *obj, int end)
   case MRB_TT_SCLASS:
     mrb_gc_free_mt(mrb, (struct RClass*)obj);
     mrb_gc_free_iv(mrb, (struct RObject*)obj);
+    mrb_mc_clear_by_class(mrb, (struct RClass*)obj);
     break;
   case MRB_TT_ICLASS:
     if (MRB_FLAG_TEST(obj, MRB_FL_CLASS_IS_ORIGIN))
       mrb_gc_free_mt(mrb, (struct RClass*)obj);
+    mrb_mc_clear_by_class(mrb, (struct RClass*)obj);
     break;
   case MRB_TT_ENV:
     {