Allow Routers to inherit state (#1368)

* Rename Fallback::Custom to Fallback::Service

* Allow Routers to inherit state

* Rename Router::{nest => nest_service} and add new nest method for Routers

* Fix lints

* Add basic tests for state inheritance

* Changelog

examples/stream-to-file/src/main.rs

@@ -131,7 +131,7 @@ where
 // to prevent directory traversal attacks we ensure the path consists of exactly one normal
 // component
 fn path_is_valid(path: &str) -> bool {
-    let path = std::path::Path::new(&*path);
+    let path = std::path::Path::new(path);
     let mut components = path.components().peekable();
 
     if let Some(first) = components.peek() {