* parser now has a callback feature for intercepting chunk headers
and chunk bodies
* The names for basic_parser derived class callbacks have been
refactored
* basic_parser offers an additional callback for distinguishing
chunk body data.
Actions Required:
* Adjust signatures for required members of user-defined
subclasses of basic_parser
* Use the new basic_parser chunk callbacks for accessing
chunk extensions and chunk bodies.
These types now perform error-based initialization in
a separate init() functions instead of in the constructor.
Actions Required:
* Modify instances of user-defined BodyReader and BodyWriter
types to perfrom two-phase initialization, as per the
updated documented type requirements.
* `parser` is now templated on Allocator
* `parser` only produces messages using `basic_fields<Allocator>`
as the Fields type.
* message-oriented read and async_read only work on messages
using basic_fields as the Fields type.
Actions Required:
* Callers using `parser` with Fields types other than basic_fields
will need to create their own subclass of basic_parser to work
with their custom fields type.
Vinnie Falco