This is a complete rework of `Responder`s and of the http backend in
general. This gets Rocket one step closer to HTTP library independence,
enabling many future features such as transparent async I/O, automatic
HEAD request parsing, pre/post hooks, and more.
Summary of changes:
* `Responder::response` no longer takes in `FreshHyperResponse`.
Instead, it returns a new `Response` type.
* The new `Response` type now encapsulates a full HTTP response. As a
result, `Responder`s now return it.
* The `Handler` type now returns an `Outcome` directly.
* The `ErrorHandler` returns a `Result`. It can no longer forward,
which made no sense previously.
* `Stream` accepts a chunked size parameter.
* `StatusCode` removed in favor of new `Status` type.
* `ContentType` significantly modified.
* New, lightweight `Header` type that plays nicely with `Response`.
