Implement Ruby2.7's frozen strings from `Module#name`

mrbgems/mruby-class-ext/test/module.rb

+def assert_module_name(exp, mod)
+  assert "assert_module_name" do
+    act = mod.__send__(:name)
+    assert_equal exp, act
+    assert_predicate act, :frozen?
+  end
+end
+
 assert 'Module#<' do
   a = Class.new
   b = Class.new(a)
@@ -58,17 +66,17 @@ assert 'Module#name' do
     const_set :SetInner, Class.new
   end
 
-  assert_equal 'Outer', Outer.name
-  assert_equal 'Outer::Inner', Outer::Inner.name
-  assert_equal 'Outer::SetInner', Outer::SetInner.name
+  assert_module_name 'Outer', Outer
+  assert_module_name 'Outer::Inner', Outer::Inner
+  assert_module_name 'Outer::SetInner', Outer::SetInner
 
   outer = Module.new do
     const_set :SetInner, Class.new
   end
   Object.const_set :SetOuter, outer
 
-  assert_equal 'SetOuter', SetOuter.name
-  assert_equal 'SetOuter::SetInner', SetOuter::SetInner.name
+  assert_module_name 'SetOuter', SetOuter
+  assert_module_name 'SetOuter::SetInner', SetOuter::SetInner
 
   mod = Module.new
   cls = Class.new

src/class.c

@@ -76,6 +76,7 @@ mrb_class_name_class(mrb_state *mrb, struct RClass *outer, struct RClass *c, mrb
     name = mrb_str_dup(mrb, name);
     mrb_str_cat_cstr(mrb, name, "::");
     mrb_str_cat_cstr(mrb, name, mrb_sym_name(mrb, id));
+    MRB_SET_FROZEN_FLAG(mrb_obj_ptr(name));
   }
   mrb_obj_iv_set_force(mrb, (struct RObject*)c, nsym, name);
 }
@@ -1712,7 +1713,7 @@ mrb_class_path(mrb_state *mrb, struct RClass *c)
     /* toplevel class/module */
     return mrb_sym_str(mrb, mrb_symbol(path));
   }
-  return mrb_str_dup(mrb, path);
+  return path;
 }
 
 MRB_API struct RClass*

src/variable.c

@@ -1123,7 +1123,7 @@ mrb_class_find_path(mrb_state *mrb, struct RClass *c)
     iv_del(mrb, c->iv, mrb_intern_lit(mrb, "__outer__"), NULL);
     iv_put(mrb, c->iv, mrb_intern_lit(mrb, "__classname__"), path);
     mrb_field_write_barrier_value(mrb, (struct RBasic*)c, path);
-    path = mrb_str_dup(mrb, path);
+    MRB_SET_FROZEN_FLAG(mrb_obj_ptr(path));
   }
   return path;
 }