* 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
This way there is now only one way to create a router:
```rust
use axum::{Router, handler::get};
let app = Router::new()
.route("/foo", get(handler))
.route("/foo", get(handler));
```
`nest` was changed in the same way:
```rust
use axum::Router;
let app = Router::new().nest("/foo", service);
```
