macros: Use a nicer syntax when declaring attributes.

The previous version had a => required|optional|default token, this was
duplicating information for Option types and didn’t look very good.

This new version looks like a type, which can be either Required<_>,
Option<_> or Default<_>, and means the same thing.

src/receipts.rs

@@ -22,7 +22,7 @@ generate_element!(
     Received, "received", RECEIPTS,
     attributes: [
         /// The 'id' attribute of the received message.
-        id: Option<String> = "id" => optional,
+        id: Option<String> = "id",
     ]
 );