Move axum-debug into axum-macros (#724)

* Move axum-debug into axum-macros

* fix ref to axum-macros in changelog

* Apply suggestions from code review

Co-authored-by: Jonas Platte <[email protected]>

Co-authored-by: Jonas Platte <[email protected]>

axum-macros/tests/debug_handler/pass/mut_extractor.rs

@@ -0,0 +1,9 @@
+use axum_macros::debug_handler;
+
+#[debug_handler]
+async fn handler(mut foo: String) -> String {
+    foo += "bar";
+    foo
+}
+
+fn main() {}