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/key-value-store/src/main.rs

@@ -26,7 +26,7 @@ use std::{
 use tower::{BoxError, ServiceBuilder};
 use tower_http::{
     auth::RequireAuthorizationLayer, compression::CompressionLayer, limit::RequestBodyLimitLayer,
-    trace::TraceLayer, ServiceBuilderExt,
+    trace::TraceLayer,
 };
 use tracing_subscriber::{layer::SubscriberExt, util::SubscriberInitExt};