Change internal role storage in Guild to a dict instead of a list.

This adds the following APIs:

* Guild.get_role

This removes the following APIs:

* Guild.role_hierarchy

To compensate for the removed APIs, Guild.roles is now a sorted list
based on hierarchy. The first element will always be the @everyone
role.

This speeds up access at the cost of some memory, theoretically.

discord/abc.py

@@ -288,8 +288,9 @@ class GuildChannel:
         """Returns a :class:`list` of :class:`Roles` that have been overridden from
         their default values in the :attr:`Guild.roles` attribute."""
         ret = []
+        g = self.guild
         for overwrite in filter(lambda o: o.type == 'role', self._overwrites):
-            role = utils.get(self.guild.roles, id=overwrite.id)
+            role = g.get_role(overwrite.id)
             if role is None:
                 continue
 
@@ -358,8 +359,7 @@ class GuildChannel:
             overwrite = PermissionOverwrite.from_pair(allow, deny)
 
             if ow.type == 'role':
-                # accidentally quadratic
-                target = utils.find(lambda r: r.id == ow.id, self.guild.roles)
+                target = self.guild.get_role(ow.id)
             elif ow.type == 'member':
                 target = self.guild.get_member(ow.id)