Refactor serialization algorithms:

serializer interface is changed to be buffer-only, no streams,
and placed in its own header file.

Operations on serializers are moved to free functions as part
of the HTTP write family of synchronous and asynchronous algorithms.

test/http/serializer.cpp

@@ -0,0 +1,9 @@
+//
+// Copyright (c) 2013-2017 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/http/serializer.hpp>