Summary of changes:
* Request no longer has a lifetime parameter.
* Handler type now includes a `Data` parameter.
* Response is now an enum that is either `Complete` or `Forward`.
* Outcome enum is now one of: Success, Failure, Forward.
* Outcome::Foward for Responses must include StatusCode.
* Responders are now final: they cannot forward to requests. (!!)
* Responsers may only forward to catchers. (!!)
* Response no longer provides wrapping methods.
* Route is now cloneable.
This change is fundamental to enabling streaming requests.
Sergio Benitez