Fix #[debug_handler] for multiple extractors (#552)

* Fix `#[debug_handler]` for multiple extractors

It generated a function for each extractor to check the type but those
functions didn't have unique names.

Fixed by including all idents in the `arg: Extractor` token tree in the
name of the function generated. Not sure there is a simpler way to fix
it.

* just use a counter

* don't need visit feature anymore

axum-debug/tests/pass/multiple_extractors.rs

@@ -0,0 +1,6 @@
+use axum_debug::debug_handler;
+
+#[debug_handler]
+async fn handler(_one: String, _two: String, _three: String) {}
+
+fn main() {}