This commit includes two major changes to core:
1. Configuration state is no longer global. The `config::active()`
function has been removed. The active configuration can be
retrieved via the `config` method on a `Rocket` instance.
2. The `Responder` trait has changed. `Responder::respond(self)` has
been removed in favor of `Responder::respond_to(self, &Request)`.
This allows responders to dynamically adjust their response based
on the incoming request.
Additionally, it includes the following changes to core and codegen:
* The `Request::guard` method was added to allow for simple
retrivial of request guards.
* The `Request::limits` method was added to retrieve configured
limits.
* The `File` `Responder` implementation now uses a fixed size body
instead of a chunked body.
* The `Outcome::of<R: Responder>(R)` method was removed while
`Outcome::from<R: Responder(&Request, R)` was added.
* The unmounted and unmanaged limits are more cautious: they will only
emit warnings when the `Rocket` receiver is known.
This commit includes one major change to contrib:
1. To use contrib's templating, the fairing returned by
`Template::fairing()` must be attached to the running Rocket
instance.
Additionally, the `Display` implementation of `Template` was removed. To
directly render a template to a `String`, the new `Template::show`
method can be used.
