Change internal representation of roles in Member and Emoji.

Introduce a new internal type, SnowflakeList, which has better memory
footprint over a regular list or set of roles. It is suspected that
there will be a 9x reduction of memory for every Emoji instance and a
48 byte saving per Member instance. However, these savings will
probably only be evident on larger bots.

As a consequence of this change, Member.roles is now computed lazily.

Currently I am not sure if I want to do the initial sorting on the
SnowflakeList for Member, as this comes with a O(n log n) cost when
creating a Member for little purpose since SnowflakeList.has is not
overly relied on. If CPU time becomes an issue this might change.

discord/role.py

@@ -173,7 +173,8 @@ class Role(Hashable):
         if self.is_default():
             return all_members
 
-        return [member for member in all_members if self in member.roles]
+        role_id = self.id
+        return [member for member in all_members if member._roles.has(role_id)]
 
     async def _move(self, position, reason):
         if position <= 0: