externals: Add mp

Merge commit '7b0c47d3f0c20eeeceb0324771eb86ab80b0bd86' as 'externals/mp'

externals/mp/tests/all_tests.cpp

@@ -0,0 +1,12 @@
+/* This file is part of the mp project.
+ * Copyright (c) 2020 MerryMage
+ * SPDX-License-Identifier: 0BSD
+ */
+
+#include "metavalue_tests.h"
+#include "traits_tests.h"
+#include "typelist_tests.h"
+
+int main() {
+    return 0;
+}