Add new function `mrb_singleton_class_ptr()`; ref #4973

The difference between `mrb_singleton_class` and `mrb_singleton_class_ptr`:

- `mrb_singleton_class_ptr` returns `struct RClass*`.
- `mrb_singleton_class_ptr` returns `NULL` on immediate values where
  `mrb_singleton_class` raises exceptions.

include/mruby.h

@@ -317,7 +317,9 @@ MRB_API struct RClass *mrb_define_class(mrb_state *mrb, const char *name, struct
  * @return [struct RClass *] Reference to the newly defined module.
  */
 MRB_API struct RClass *mrb_define_module(mrb_state *mrb, const char *name);
+
 MRB_API mrb_value mrb_singleton_class(mrb_state *mrb, mrb_value val);
+MRB_API struct RClass *mrb_singleton_class_ptr(mrb_state *mrb, mrb_value val);
 
 /**
  * Include a module in another class or module.

src/class.c

@@ -1242,33 +1242,45 @@ mrb_mod_dummy_visibility(mrb_state *mrb, mrb_value mod)
   return mod;
 }
 
-MRB_API mrb_value
-mrb_singleton_class(mrb_state *mrb, mrb_value v)
+/* returns mrb_class_ptr(mrb_singleton_class()) */
+/* except that it return NULL for immediate values */
+MRB_API struct RClass*
+mrb_singleton_class_ptr(mrb_state *mrb, mrb_value v)
 {
   struct RBasic *obj;
 
   switch (mrb_type(v)) {
   case MRB_TT_FALSE:
     if (mrb_nil_p(v))
-      return mrb_obj_value(mrb->nil_class);
-    return mrb_obj_value(mrb->false_class);
+      return mrb->nil_class;
+    return mrb->false_class;
   case MRB_TT_TRUE:
-    return mrb_obj_value(mrb->true_class);
+    return mrb->true_class;
   case MRB_TT_CPTR:
-    return mrb_obj_value(mrb->object_class);
+    return mrb->object_class;
   case MRB_TT_SYMBOL:
   case MRB_TT_FIXNUM:
 #ifndef MRB_WITHOUT_FLOAT
   case MRB_TT_FLOAT:
 #endif
-    mrb_raise(mrb, E_TYPE_ERROR, "can't define singleton");
-    return mrb_nil_value();    /* not reached */
+    return NULL;
   default:
     break;
   }
   obj = mrb_basic_ptr(v);
   prepare_singleton_class(mrb, obj);
-  return mrb_obj_value(obj->c);
+  return obj->c;
+}
+
+MRB_API mrb_value
+mrb_singleton_class(mrb_state *mrb, mrb_value v)
+{
+  struct RClass *c = mrb_singleton_class_ptr(mrb, v);
+
+  if (c == NULL) {
+    mrb_raise(mrb, E_TYPE_ERROR, "can't define singleton");
+  }
+  return mrb_obj_value(c);
 }
 
 MRB_API void