Implement AutoShardedClient for transparent sharding.

This allows people to run their >2,500 guild bot in a single process
without the headaches of IPC/RPC or much difficulty.

discord/guild.py

@@ -324,6 +324,14 @@ class Guild(Hashable):
         """Returns the true member count regardless of it being loaded fully or not."""
         return self._member_count
 
+    @property
+    def shard_id(self):
+        """Returns the shard ID for this guild if applicable."""
+        count = self._state.shard_count
+        if count is None:
+            return None
+        return (self.id >> 22) % count
+
     @property
     def created_at(self):
         """Returns the guild's creation time in UTC."""