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);
```
This breaks up `extract.rs` into several smaller submodules. The public
API remains the same.
This is done in prep for adding more tests to extractors which would get
messy if they were all in the same file.
David Pedersen