Tidy up core sources:

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

include/beast/core/streambuf.hpp

@@ -0,0 +1,28 @@
+//
+// 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_STREAMBUF_HPP
+#define BEAST_STREAMBUF_HPP
+
+#include <beast/core/basic_streambuf.hpp>
+
+namespace beast {
+
+/** A @b `Streambuf` that uses multiple buffers internally.
+
+    The implementation uses a sequence of one or more character arrays
+    of varying sizes. Additional character array objects are appended to
+    the sequence to accommodate changes in the size of the character
+    sequence.
+
+    @note Meets the requirements of @b `Streambuf`.
+*/
+using streambuf = basic_streambuf<std::allocator<char>>;
+
+} // beast
+
+#endif