Refactor HTTP serialization (API Change):

A new class `serializer` is introduced to permit incremental
serialization of HTTP messages. Existing free functions are
re-implemented in terms of this new class.

* The BodyReader concept is refined to support a greater variety
of strategies for providing buffers representing the body to
the serialization algorithms.

* Added buffer_body, a new model of Body which allows the caller
to provide a series of owned buffers using their own serialization
loop.

* Added empty_body, a model of Body which is for serialization only,
to represent HTTP messages with no content body.

* Removed overloads of write and async_write which send only
the HTTP header.

* Removed public interfaces for performing low-level chunk encoding.

include/beast/websocket/impl/stream.ipp

@@ -196,10 +196,10 @@ build_request(detail::sec_ws_key_type& key,
 }
 
 template<class NextLayer>
-template<class Decorator>
+template<class Fields, class Decorator>
 response_type
 stream<NextLayer>::
-build_response(request_type const& req,
+build_response(http::header<true, Fields> const& req,
     Decorator const& decorator)
 {
     auto const decorate =