Always store state in an Arc (#1270)

* Add extension and state benchmarks

* wip

* Arc the state everywhere

* don't require `S: Clone`

* fix example

examples/customize-path-rejection/src/main.rs

@@ -57,7 +57,7 @@ where
     // these trait bounds are copied from `impl FromRequest for axum::extract::path::Path`
     T: DeserializeOwned + Send,
     B: Send,
-    S: Send,
+    S: Send + Sync,
 {
     type Rejection = (StatusCode, axum::Json<PathError>);