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/member.py

@@ -189,8 +189,8 @@ class Member(discord.abc.Messageable, _BaseUser):
     def _update_roles(self, data):
         # update the roles
         self.roles = [self.guild.default_role]
-        for roleid in map(int, data['roles']):
-            role = utils.find(lambda r: r.id == roleid, self.guild.roles)
+        for role_id in map(int, data['roles']):
+            role = self.guild.get_role(role_id)
             if role is not None:
                 self.roles.append(role)