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.
