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

axum-macros/tests/from_ref/pass/basic.rs

@@ -1,5 +1,4 @@
-use axum_macros::FromRef;
-use axum::{Router, routing::get, extract::State};
+use axum::{Router, routing::get, extract::{State, FromRef}};
 
 // This will implement `FromRef` for each field in the struct.
 #[derive(Clone, FromRef)]
@@ -15,5 +14,7 @@ fn main() {
         auth_token: Default::default(),
     };
 
-    let _: Router<AppState> = Router::with_state(state).route("/", get(handler));
+    let _: axum::routing::RouterService = Router::new()
+        .route("/", get(handler))
+        .with_state(state);
 }