Tidy up core sources:

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

include/beast/core/to_string.hpp

@@ -0,0 +1,51 @@
+//
+// 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_TO_STRING_HPP
+#define BEAST_TO_STRING_HPP
+
+#include <beast/core/buffer_concepts.hpp>
+#include <boost/asio/buffer.hpp>
+#include <string>
+
+namespace beast {
+
+/** Convert a @b `ConstBufferSequence` to a `std::string`.
+
+    This function will convert the octets in a buffer sequence to a string.
+    All octets will be inserted into the resulting string, including null
+    or unprintable characters.
+
+    @param buffers The buffer sequence to convert.
+
+    @return A string representing the contents of the input area.
+
+    @note This function participates in overload resolution only if
+    the streambuf parameter meets the requirements of @b `Streambuf`.
+*/
+template<class ConstBufferSequence
+#if ! GENERATING_DOCS
+    ,class = std::enable_if<is_ConstBufferSequence<
+        ConstBufferSequence>::value>
+#endif
+>
+std::string
+to_string(ConstBufferSequence const& buffers)
+{
+    using boost::asio::buffer_cast;
+    using boost::asio::buffer_size;
+    std::string s;
+    s.reserve(buffer_size(buffers));
+    for(auto const& buffer : buffers)
+        s.append(buffer_cast<char const*>(buffer),
+            buffer_size(buffer));
+    return s;
+}
+
+} // beast
+
+#endif