Tidy up core sources:

The core headers are moved to their own directory (but remain in
the same namespace).

test/core/handler_alloc.cpp

@@ -0,0 +1,28 @@
+//
+// Copyright (c) 2013-2016 Vinnie Falco (vinnie dot falco at gmail dot 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)
+//
+
+// Test that header file is self-contained.
+#include <beast/core/to_string.hpp>
+
+#include <beast/unit_test/suite.hpp>
+#include <boost/asio/buffer.hpp>
+
+namespace beast {
+
+class to_string_test : public beast::unit_test::suite
+{
+public:
+    void run()
+    {
+        expect(to_string(boost::asio::const_buffers_1("x", 1)) == "x");
+    }
+};
+
+BEAST_DEFINE_TESTSUITE(to_string,core,beast);
+
+} // beast
+