Make axum-debug handle more cases (#518)

* Make `axum-debug` handle more cases

* Only just trybuild tests on stable

* revert changes to hello-world example

* Remove a bit of duplication

* return error on generics

* address review feedback

* Support associated functions with receiver or returns `Self`

* fix indentation

axum-debug/tests/pass/impl_future.rs

@@ -0,0 +1,9 @@
+use axum_debug::debug_handler;
+use std::future::Future;
+
+#[debug_handler]
+fn handler() -> impl Future<Output = ()> {
+    async {}
+}
+
+fn main() {}