This commit is a codebase-wide cleanup driven by clippy warnings. In
addition to fixing every reasonable warning, the following new
functionality was introduced:
* `Accept::new()` now takes any `T: Into<QMediaType>` iterator.
* `TempFile::is_empty()` was added.
* `HeaderMap` now implements `IntoIterator`.
This commit makes the following breaking changes:
* The `IntoCollection` trait is gone. Generics previously bound by the
trait are now bound by `IntoIterator`. This affects:
- `Accept::new()`
- `ContentType::with_params()`
- `Permission::{allow, allowed}()`
* `MediaType`, `QMediaType`, and `Allow` implement `IntoIterator`,
enabling most existing code to continue working without change.
* The inherent `HeaderMap::into_iter()` method was removed.
* The `Ok` variant in ErrorKind::Liftoff` is now `Box<Rocket<Orbit>>`.
Sergio Benitez
Arjen