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/concepts.cpp

@@ -7,3 +7,15 @@
 
 // Test that header file is self-contained.
 #include <beast/http/concepts.hpp>
+
+#include <beast/http/empty_body.hpp>
+
+namespace beast {
+namespace http {
+
+BOOST_STATIC_ASSERT(! is_Writer<int>::value);
+
+BOOST_STATIC_ASSERT(is_Writer<empty_body>::value);
+
+} // http
+} // beast