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/sasl.rs

@@ -49,7 +49,7 @@ generate_element!(
     Auth, "auth", SASL,
     attributes: [
         /// The mechanism used.
-        mechanism: Mechanism = "mechanism" => required
+        mechanism: Required<Mechanism> = "mechanism"
     ],
     text: (
         /// The content of the handshake.