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/eme.rs

@@ -11,11 +11,11 @@ generate_element!(
     ExplicitMessageEncryption, "encryption", EME,
     attributes: [
         /// Namespace of the encryption scheme used.
-        namespace: String = "namespace" => required,
+        namespace: Required<String> = "namespace",
 
         /// User-friendly name for the encryption scheme, should be `None` for OTR,
         /// legacy OpenPGP and OX.
-        name: Option<String> = "name" => optional,
+        name: Option<String> = "name",
     ]
 );