Change certain sequences to use a special proxy type instead of list

This is to speed up cases where someone is just querying the length
of the underlying sequence. If anything else is done to the sequence
then it is copied from the original iterator.

This change should be mostly transparent.

discord/role.py

@@ -347,7 +347,7 @@ class Role(Hashable):
     @property
     def members(self) -> List[Member]:
         """List[:class:`Member`]: Returns all the members with this role."""
-        all_members = self.guild.members
+        all_members = list(self.guild._members.values())
         if self.is_default():
             return all_members