Refactor chunked-encoding serialization:

New buffer sequence classes are provided to allow full
control over the serialization of chunk-encoded message
payloads:

* chunk_header

    A ConstBufferSequence representing the chunk header.
    It includes a hexadecimal-encoded size, an optional
    set of chunk extensions, and the trailing CRLF
    required to denote the end of the chunk header.

    This allows the caller to manually serialize the chunk
    body in one or more calls to a stream output function.
    The caller must also output an object of type `chunk_crlf`
    to the stream to indicate the end of the chunk body.

* chunk_crlf

    A small ConstBufferSequence that simply represents
    the two character sequence "\r\n" (CRLF). This is needed
    for the case where the caller wants to output a chunk
    body as a series of buffers (i.e. "chunking a chunk").

* chunk_body

    A ConstBufferSequence representing a complete chunk.
    This includes the size, an optional set of chunk extensions,
    a caller provided buffer containing the body, and the
    required CRLF that follows.

* chunk_final

    A ConstBufferSequence representing a final chunk. It
    includes an optional set of caller-provided field trailers

* chunk_extensions

    A container for building a set of chunk extensions to use
    during serialization. The use of the container is optional,
    callers may provide their own buffer containing a correctly
    formatted set of chunk extensions, or they may use their
    own convenience container which meets the requirements.

The basic_fields container is modified to allow construction
outside the context of a message. The container can be used
to provide trailers to `chunk_final`.

Actions Required:

* Remove references to ChunkDecorators. Use the new chunk-encoding
  buffer sequences to manually produce a chunked payload body in
  the case where control over the chunk-extensions and/or trailers
  is required.

example/common/write_msg.hpp

@@ -170,9 +170,9 @@ public:
     `async_write_some` function, and is known as a <em>composed operation</em>.
     The program must ensure that the stream performs no other write operations
     until this operation completes. The algorithm will use a temporary
-    @ref serializer with an empty chunk decorator to produce buffers. If
-    the semantics of the message indicate that the connection should be
-    closed after the message is sent, the error delivered by this function
+    @ref serializer to produce buffers. If the semantics of the message
+    indicate that the connection should be closed after the message is sent,
+    the error delivered by this function
     will be @ref error::end_of_stream
 
     @param stream The stream to which the data is to be written.