Refactor memory resource includes

test/Jamfile

@@ -31,6 +31,7 @@ local SOURCES =
     except.cpp
     json.cpp
     kind.cpp
+    memory_resource.cpp
     monotonic_resource.cpp
     number_cast.cpp
     object.cpp

test/memory_resource.cpp

@@ -0,0 +1,11 @@
+//
+// Copyright (c) 2019 Vinnie Falco (vinnie.falco@gmail.com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+// Official repository: https://github.com/vinniefalco/json
+//
+
+// Test that header file is self-contained.
+#include <boost/json/memory_resource.hpp>