It cleans up the docs to have the futures in their own modules as users
are unlikely to look at them. Also matches the pattern used in tower
https://docs.rs/tower/0.4.8/tower/util/future/index.html.
Added re-exports to the old locations so its not a breaking change.
Previously extractors worked directly on `Request<B>` which meant you
had to do weird tricks like `mem::take(req.headers_mut())` to get owned
parts of the request.
This changes that instead to use a new `RequestParts` type that have
methods to "take" each part of the request. Without having to do weird
tricks.
Also removed the need to have `B: Default` for body extractors.
David Pedersen