variable.c: add `skip` argument to skip `base` class in lookup.

`mrb_vm_const_get` function looks up the constant first in the base
class, so that fallback `const_get` need not to search from the base.

src/variable.c

@@ -766,13 +766,14 @@ mod_const_check(mrb_state *mrb, mrb_value mod)
 }
 
 static mrb_value
-const_get(mrb_state *mrb, struct RClass *base, mrb_sym sym)
+const_get(mrb_state *mrb, struct RClass *base, mrb_sym sym, mrb_bool skip)
 {
   struct RClass *c = base;
   mrb_value v;
   mrb_bool retry = FALSE;
   mrb_value name;
 
+  if (skip) c = c->super;
 L_RETRY:
   while (c) {
     if (!MRB_FLAG_TEST(c, MRB_FL_CLASS_IS_PREPENDED) && c->iv) {
@@ -794,7 +795,7 @@ MRB_API mrb_value
 mrb_const_get(mrb_state *mrb, mrb_value mod, mrb_sym sym)
 {
   mod_const_check(mrb, mod);
-  return const_get(mrb, mrb_class_ptr(mod), sym);
+  return const_get(mrb, mrb_class_ptr(mod), sym, FALSE);
 }
 
 mrb_value
@@ -829,7 +830,7 @@ mrb_vm_const_get(mrb_state *mrb, mrb_sym sym)
     }
     proc = proc->upper;
   }
-  return const_get(mrb, c, sym);
+  return const_get(mrb, c, sym, TRUE);
 }
 
 MRB_API void