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.

examples/global-404-handler/src/main.rs

@@ -24,8 +24,8 @@ async fn main() {
     // build our application with a route
     let app = Router::new().route("/", get(handler));
 
-    // make sure this is added as the very last thing
-    let app = app.or(handler_404.into_service());
+    // add a fallback service for handling routes to unknown paths
+    let app = app.fallback(handler_404.into_service());
 
     // run it
     let addr = SocketAddr::from(([127, 0, 0, 1], 3000));