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/fail/extract_self_ref.rs

@@ -0,0 +1,23 @@
+use axum::{
+    async_trait,
+    extract::{FromRequest, RequestParts},
+};
+use axum_debug::debug_handler;
+
+struct A;
+
+#[async_trait]
+impl FromRequest for A {
+    type Rejection = ();
+
+    async fn from_request(_req: &mut RequestParts) -> Result<Self, Self::Rejection> {
+        unimplemented!()
+    }
+}
+
+impl A {
+    #[debug_handler]
+    async fn handler(&self) {}
+}
+
+fn main() {}