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.

test/Jamfile

@@ -42,6 +42,7 @@ unit-test core-tests :
 unit-test http-tests :
     ../extras/beast/unit_test/main.cpp
     http/basic_parser.cpp
+    http/buffer_body.cpp
     http/concepts.cpp
     http/design.cpp
     http/dynamic_body.cpp
@@ -54,7 +55,6 @@ unit-test http-tests :
     http/rfc7230.cpp
     http/string_body.cpp
     http/write.cpp
-    http/chunk_encode.cpp
     ;
 
 unit-test http-bench :