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/message_correct.rs

@@ -12,7 +12,7 @@ generate_element!(
     Replace, "replace", MESSAGE_CORRECT,
     attributes: [
         /// The 'id' attribute of the message getting corrected.
-        id: String = "id" => required,
+        id: Required<String> = "id",
     ]
 );