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/http/basic_parser.cpp

@@ -13,6 +13,7 @@
 #include <beast/core/buffer_cat.hpp>
 #include <beast/core/consuming_buffers.hpp>
 #include <beast/core/multi_buffer.hpp>
+#include <beast/core/ostream.hpp>
 #include <beast/unit_test/suite.hpp>
 
 namespace beast {
@@ -903,7 +904,7 @@ public:
     {
 #if 0
         multi_buffer b;
-        b << 
+        ostream(b) << 
             "POST / HTTP/1.1\r\n"
             "Content-Length: 5\r\n"
             "\r\n"