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/chunk_encode.hpp

@@ -1,75 +0,0 @@
-//
-// Copyright (c) 2013-2017 Vinnie Falco (vinnie dot falco at gmail dot com)
-//
-// Distributed under the Boost Software License, Version 1.0. (See accompanying
-// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
-//
-
-#ifndef BEAST_HTTP_CHUNK_ENCODE_HPP
-#define BEAST_HTTP_CHUNK_ENCODE_HPP
-
-#include <beast/config.hpp>
-#include <beast/core/buffer_cat.hpp>
-#include <beast/http/detail/chunk_encode.hpp>
-#include <boost/asio/buffer.hpp>
-#include <boost/assert.hpp>
-#include <algorithm>
-#include <array>
-#include <cstddef>
-#include <iterator>
-#include <type_traits>
-
-namespace beast {
-namespace http {
-
-/** Returns a chunk-encoded ConstBufferSequence.
-
-    This returns a buffer sequence representing the
-    first chunk of a chunked transfer coded body.
-
-    @param fin `true` if this is the last chunk.
-
-    @param buffers The input buffer sequence.
-
-    @return A chunk-encoded ConstBufferSequence representing the input.
-
-    @see <a href=https://tools.ietf.org/html/rfc7230#section-4.1.3>rfc7230 section 4.1.3</a>
-*/
-template<class ConstBufferSequence>
-#if BEAST_DOXYGEN
-implementation_defined
-#else
-beast::buffers_view<
-    detail::chunk_encode_delim,
-    ConstBufferSequence,
-    boost::asio::const_buffers_1>
-#endif
-chunk_encode(bool fin, ConstBufferSequence const& buffers)
-{
-    using boost::asio::buffer_size;
-    return buffer_cat(
-        detail::chunk_encode_delim{buffer_size(buffers)},
-        buffers,
-        fin ? boost::asio::const_buffers_1{"\r\n0\r\n\r\n", 7}
-            : boost::asio::const_buffers_1{"\r\n", 2});
-}
-
-/** Returns a chunked encoding final chunk.
-
-    @see <a href=https://tools.ietf.org/html/rfc7230#section-4.1.3>rfc7230 section 4.1.3</a>
-*/
-inline
-#if BEAST_DOXYGEN
-implementation_defined
-#else
-boost::asio::const_buffers_1
-#endif
-chunk_encode_final()
-{
-    return boost::asio::const_buffers_1{"0\r\n\r\n", 5};
-}
-
-} // http
-} // beast
-
-#endif