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/http/impl/error.ipp

@@ -41,6 +41,7 @@ public:
         default:
         case error::end_of_stream: return "end of stream";
         case error::partial_message: return "partial message";
+        case error::need_more: return "need more";
         case error::buffer_overflow: return "buffer overflow";
         case error::bad_line_ending: return "bad line ending";
         case error::bad_method: return "bad method";

include/beast/http/impl/message.ipp

@@ -163,8 +163,7 @@ prepare(message<isRequest, Body, Fields>& msg,
         "Body requirements not met");
     static_assert(has_writer<Body>::value,
         "Body has no writer");
-    static_assert(is_Writer<typename Body::writer,
-        message<isRequest, Body, Fields>>::value,
+    static_assert(is_Writer<Body>::value,
             "Writer requirements not met");
     detail::prepare_info pi;
     detail::prepare_content_length(pi, msg,