Type safe state inheritance (#1532)

* Make state type safe

* fix examples

* remove unnecessary `#[track_caller]`s

* Router::into_service -> Router::with_state

* fixup docs

* macro docs

* add missing docs

* fix examples

* format

* changelog

* Update trybuild tests

* Make sure fallbacks are still inherited for opaque services (#1540)

* Document nesting routers with different state

* fix leftover conflicts

examples/error-handling-and-dependency-injection/src/main.rs

@@ -36,9 +36,10 @@ async fn main() {
     let user_repo = Arc::new(ExampleUserRepo) as DynUserRepo;
 
     // Build our application with some routes
-    let app = Router::with_state(user_repo)
+    let app = Router::new()
         .route("/users/:id", get(users_show))
-        .route("/users", post(users_create));
+        .route("/users", post(users_create))
+        .with_state(user_repo);
 
     // Run our application
     let addr = SocketAddr::from(([127, 0, 0, 1], 3000));