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/abc.py

@@ -415,9 +415,10 @@ class GuildChannel:
 
         default = self.guild.default_role
         base = Permissions(default.permissions.value)
+        roles = member.roles
 
         # Apply guild roles that the member has.
-        for role in member.roles:
+        for role in roles:
             base.value |= role.permissions.value
 
         # Guild-wide Administrator -> True for everything
@@ -436,7 +437,13 @@ class GuildChannel:
         except IndexError:
             remaining_overwrites = self._overwrites
 
-        member_role_ids = set(map(lambda r: r.id, member.roles))
+        # not sure if doing member._roles.get(...) is better than the
+        # set approach. While this is O(N) to re-create into a set for O(1)
+        # the direct approach would just be O(log n) for searching with no
+        # extra memory overhead. For now, I'll keep the set cast
+        # Note that the member.roles accessor up top also creates a
+        # temporary list
+        member_role_ids = {r.id for r in roles}
         denies = 0
         allows = 0