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/error.hpp

@@ -33,6 +33,17 @@ enum class error
     */
     partial_message,
 
+    /** Additional buffers are required.
+
+        This error is generated during serialization of HTTP
+        messages using the @ref buffer_body representation.
+        It indicates to the caller that an additional buffer
+        sequence should be placed into the body, or that the
+        caller should indicate that there are no more bytes
+        remaining in the body to serialize.
+    */
+    need_more,
+
     /** Buffer maximum exceeded.
 
         This error is returned when reading HTTP content