Tidy up core sources:

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

include/beast/websocket/impl/read_frame_op.ipp

@@ -9,9 +9,9 @@
 #define BEAST_WEBSOCKET_IMPL_READ_FRAME_OP_HPP
 
 #include <beast/websocket/teardown.hpp>
-#include <beast/handler_alloc.hpp>
-#include <beast/prepare_buffers.hpp>
-#include <beast/static_streambuf.hpp>
+#include <beast/core/handler_alloc.hpp>
+#include <beast/core/prepare_buffers.hpp>
+#include <beast/core/static_streambuf.hpp>
 #include <boost/optional.hpp>
 #include <cassert>
 #include <memory>