Refactor proc-macro attribute parsing (#1369)

* Refactor proc-macro attribute parsing

* Remove `#[allow(warnings)]` which was accidentally committed

* Change span for "cannot use `rejection` without `via`" error for enums

* fix test

axum-macros/src/lib.rs

@@ -47,6 +47,7 @@ use proc_macro::TokenStream;
 use quote::{quote, ToTokens};
 use syn::parse::Parse;
 
+mod attr_parsing;
 mod debug_handler;
 mod from_request;
 mod typed_path;