Fix warnings emitted with -Wall

(cherry picked from commit 8c6cc1ec15)

src/core/class_db.cpp

@@ -388,7 +388,7 @@ void ClassDB::deinitialize(GDExtensionInitializationLevel p_level) {
 		{
 			std::lock_guard<std::mutex> lock(engine_singletons_mutex);
 			singleton_objects.reserve(engine_singletons.size());
-			for (const std::pair<StringName, Object *> &pair : engine_singletons) {
+			for (const std::pair<const StringName, Object *> &pair : engine_singletons) {
 				singleton_objects.push_back(pair.second);
 			}
 		}

src/core/object.cpp

@@ -75,7 +75,7 @@ MethodInfo::operator Dictionary() const {
 	dict["name"] = name;
 	dict["args"] = internal::convert_property_list(arguments);
 	Array da;
-	for (int i = 0; i < default_arguments.size(); i++) {
+	for (size_t i = 0; i < default_arguments.size(); i++) {
 		da.push_back(default_arguments[i]);
 	}
 	dict["default_args"] = da;