* Add `middleware::from_fn` for creating middleware from async fns
* More trait impls for `Next`
* Make `Next::run` consume `self`
* Use `.router_layer` in example, since middleware returns early
* Actually `Next` probably shouldn't impl `Clone` and `Service`
Has implications for backpressure and stuff
* Simplify `print-request-response` example
* Address review feedback
* add changelog link
* Move `axum-handle-error-extract` into axum
With 0.4 underway we can now nuke `axum-handle-error-extract` and move
its code directly into axum.
So this replaces the old `HandleErrorLayer` with one that supports async
functions and extractors.
* changelog
* fix CI
