This is a breaking change. It modifies collisions with respect to query
parameters as well as the default ranking of routes.
A route that does not specify query parameters will now match against
requests with _and without_ query parameters, assuming all other
elements of the route match as well. A route that _does_ specify query
parameters will only match requests with query parameters; this remains
true.
To accommodate this change in the most natural manner possible, the
default rankings of routes have changed as illustrated below:
|-------------+-------+----------+---------------|
| static path | query | new rank | previous rank |
|-------------+-------+----------+---------------|
| yes | yes | -4 | 0 |
| yes | no | -3 | 0 |
| no | yes | -2 | 1 |
| no | no | -1 | 1 |
|-------------+-------+----------+---------------|
In other words, the most specific routes, with preference for paths over
queries, are ranked highest (lower number).
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`.
Experimented with the new impl specialization features of Rust. They work! But
they're not quite there yet. Specifically, I was able to specialize on
`Responder`, but when trying to remove the macro in `FromParam`, it didn't work.
See https://github.com/rust-lang/rust/issues/31844.
