Tidy up core sources:

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

include/beast/core/write_streambuf.hpp

@@ -0,0 +1,63 @@
+//
+// 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_WRITE_STREAMBUF_HPP
+#define BEAST_WRITE_STREAMBUF_HPP
+
+#include <beast/core/buffer_concepts.hpp>
+#include <beast/core/detail/write_streambuf.hpp>
+#include <type_traits>
+#include <utility>
+
+namespace beast {
+
+/** Write to a Streambuf.
+
+    This function appends the serialized representation of each provided
+    argument into the stream buffer. It is capable of converting the
+    following types of arguments:
+
+    @li `boost::asio::const_buffer`
+
+    @li `boost::asio::mutable_buffer`
+
+    @li A type meeting the requirements of @b `ConvertibleToConstBuffer`
+
+    @li A type meeting the requirements of @b `ConstBufferSequence`
+
+    @li A type meeting the requirements of @b `MutableBufferSequence`
+
+    For all types not listed above, the function will invoke
+    `boost::lexical_cast` on the argument in an attempt to convert to
+    a string, which is then appended to the stream buffer.
+
+    When this function serializes numbers, it converts them to
+    their text representation as if by a call to `std::to_string`.
+
+    @param streambuf The stream buffer to write to.
+
+    @param args A list of one or more arguments to write.
+
+    @throws unspecified Any exceptions thrown by `boost::lexical_cast`.
+
+    @note This function participates in overload resolution only if
+    the `streambuf` parameter meets the requirements of @b `Streambuf`.
+*/
+template<class Streambuf, class... Args>
+#if GENERATING_DOCS
+void
+#else
+typename std::enable_if<is_Streambuf<Streambuf>::value>::type
+#endif
+write(Streambuf& streambuf, Args const&... args)
+{
+    detail::write_streambuf(streambuf, args...);
+}
+
+} // beast
+
+#endif