With https://github.com/tokio-rs/axum/pull/404 and https://github.com/tokio-rs/axum/pull/402 all routes now have the same types and thus we don't need to nest them but can instead store them all in a map. This simplifies the routing quite a bit and is faster as well.
High level changes:
- Routes are now stored in a `HashMap<RouteId, Route<B>>`.
- `Router::or` is renamed to `Router::merge` because thats what it does now. It copies all routes from one router to another. This also means overlapping routes will cause a panic which is nice win.
- `Router::merge` now only accepts `Router`s so added `Router::fallback` for adding a global 404 handler.
- The `Or` service has been removed.
- `Router::layer` now only adds layers to the routes you actually have meaning middleware runs _after_ routing. I believe that addresses https://github.com/tokio-rs/axum/issues/380 but will test that on another branch.
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);
```
