Change Router::with_state and impl Service for Router<()> (#1552)

* Implement `Service` for `Router<(), B>`

* wip

* wip

* fix some tests

* fix examples

* fix doc tests

* clean up docs

* changelog

* fix

* also call `with_state` when converting `MethodRouter` into a `MakeService`

* suggestions from review

examples/simple-router-wasm/src/main.rs

@@ -40,8 +40,7 @@ fn main() {
 
 #[allow(clippy::let_and_return)]
 async fn app(request: Request<String>) -> Response {
-    let mut router = Router::new().route("/api/", get(index)).into_service();
-
+    let mut router = Router::new().route("/api/", get(index));
     let response = router.call(request).await.unwrap();
     response
 }