Stores routes in a map (#408)

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.

src/tests/mod.rs

@@ -31,11 +31,12 @@ use tower_service::Service;
 
 pub(crate) use helpers::*;
 
+mod fallback;
 mod get_to_head;
 mod handle_error;
 mod helpers;
+mod merge;
 mod nest;
-mod or;
 
 #[tokio::test]
 async fn hello_world() {