Tidy up core sources:

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

include/beast/core/buffer_cat.hpp

@@ -0,0 +1,53 @@
+//
+// 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)
+//
+
+#ifndef BEAST_BUFFER_CAT_HPP
+#define BEAST_BUFFER_CAT_HPP
+
+#include <beast/core/detail/buffer_cat.hpp>
+#include <boost/asio/buffer.hpp>
+#include <cstdint>
+#include <iterator>
+#include <new>
+#include <stdexcept>
+#include <tuple>
+#include <utility>
+
+namespace beast {
+
+/** Concatenate 2 or more buffer sequences to form a `ConstBufferSequence`.
+
+    This function returns a @b `ConstBufferSequence` that when iterated,
+    efficiently concatenates the input buffer sequences. Copies of the
+    arguments passed will be made; however, the returned object does
+    not take ownership of the underlying memory. The application is still
+    responsible for managing the lifetime of the referenced memory.
+
+    @param buffers The list of buffer sequences to concatenate.
+
+    @return A new @b `ConstBufferSequence` that represents the
+    concatenation of the input buffer sequences.
+*/
+#if GENERATING_DOCS
+template<class... BufferSequence>
+implementation_defined
+buffer_cat(BufferSequence const&... buffers)
+#else
+template<class B1, class B2, class... Bn>
+detail::buffer_cat_helper<
+    boost::asio::const_buffer, B1, B2, Bn...>
+buffer_cat(B1 const& b1, B2 const& b2, Bn const&... bn)
+#endif
+{
+    return detail::buffer_cat_helper<
+        boost::asio::const_buffer,
+            B1, B2, Bn...>(b1, b2, bn...);
+}
+
+} // beast
+
+#endif