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/muc/muc.rs

@@ -14,16 +14,16 @@ generate_element!(
     History, "history", MUC,
     attributes: [
         /// How many characters of history to send, in XML characters.
-        maxchars: Option<u32> = "maxchars" => optional,
+        maxchars: Option<u32> = "maxchars",
 
         /// How many messages to send.
-        maxstanzas: Option<u32> = "maxstanzas" => optional,
+        maxstanzas: Option<u32> = "maxstanzas",
 
         /// Only send messages received in these last seconds.
-        seconds: Option<u32> = "seconds" => optional,
+        seconds: Option<u32> = "seconds",
 
         /// Only send messages after this date.
-        since: Option<DateTime> = "since" => optional,
+        since: Option<DateTime> = "since",
     ]
 );