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_request/fail/parts_extracting_body.rs

@@ -1,5 +1,4 @@
 use axum::{extract::FromRequestParts, response::Response};
-use axum_macros::FromRequestParts;
 
 #[derive(FromRequestParts)]
 struct Extractor {

axum-macros/tests/from_request/fail/parts_extracting_body.stderr

@@ -1,7 +1,7 @@
 error[E0277]: the trait bound `String: FromRequestParts<S>` is not satisfied
- --> tests/from_request/fail/parts_extracting_body.rs:6:11
+ --> tests/from_request/fail/parts_extracting_body.rs:5:11
   |
-6 |     body: String,
+5 |     body: String,
   |           ^^^^^^ the trait `FromRequestParts<S>` is not implemented for `String`
   |
   = help: the following other types implement trait `FromRequestParts<S>`: