externals: Add mcl v0.1.3

Merge commit '7eb1d05f63c7ba8df6a203138932ea428ab4aa49' as 'externals/mcl'

externals/mcl/src/assert.cpp

@@ -0,0 +1,20 @@
+// This file is part of the mcl project.
+// Copyright (c) 2022 merryhime
+// SPDX-License-Identifier: MIT
+
+#include "mcl/assert.hpp"
+
+#include <cstdio>
+#include <exception>
+
+#include <fmt/format.h>
+
+namespace mcl::detail {
+
+[[noreturn]] void assert_terminate_impl(fmt::string_view msg, fmt::format_args args) {
+    fmt::print(stderr, "assertion failed: ");
+    fmt::vprint(stderr, msg, args);
+    std::terminate();
+}
+
+}  // namespace mcl::detail