Tidy up core sources:

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

include/beast/core/buffer_concepts.hpp

@@ -0,0 +1,61 @@
+//
+// 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_CONCEPTS_HPP
+#define BEAST_BUFFER_CONCEPTS_HPP
+
+#include <beast/core/detail/buffer_concepts.hpp>
+#include <boost/asio/buffer.hpp>
+#include <type_traits>
+
+namespace beast {
+
+/// Determine if `T` meets the requirements of @b `BufferSequence`.
+template<class T, class BufferType>
+#if GENERATING_DOCS
+struct is_BufferSequence : std::integral_constant<bool, ...>
+#else
+struct is_BufferSequence : detail::is_BufferSequence<T, BufferType>::type
+#endif
+{
+};
+
+/// Determine if `T` meets the requirements of @b `ConstBufferSequence`.
+template<class T>
+#if GENERATING_DOCS
+struct is_ConstBufferSequence : std::integral_constant<bool, ...>
+#else
+struct is_ConstBufferSequence :
+    is_BufferSequence<T, boost::asio::const_buffer>
+#endif
+{
+};
+
+/// Determine if `T` meets the requirements of @b `MutableBufferSequence`.
+template<class T>
+#if GENERATING_DOCS
+struct is_MutableBufferSequence : std::integral_constant<bool, ...>
+#else
+struct is_MutableBufferSequence :
+    is_BufferSequence<T, boost::asio::mutable_buffer>
+#endif
+{
+};
+
+/// Determine if `T` meets the requirements of @b `Streambuf`.
+template<class T>
+#if GENERATING_DOCS
+struct is_Streambuf : std::integral_constant<bool, ...>
+#else
+struct is_Streambuf : detail::is_Streambuf<T>::type
+#endif
+{
+};
+
+} // beast
+
+#endif