Change CHANNEL_PINS_UPDATE to use guild information from gateway

This changes the channel lookup from O(n) to two amortised O(1) lookups

discord/state.py

@@ -702,19 +702,21 @@ class ConnectionState:
 
     def parse_channel_pins_update(self, data):
         channel_id = int(data['channel_id'])
-        channel = self.get_channel(channel_id)
+        try:
+            guild = self._get_guild(int(data['guild_id']))
+        except KeyError:
+            guild = None
+            channel = self._get_private_channel(channel_id)
+        else:
+            channel = guild and guild._resolve_channel(channel_id)
+
         if channel is None:
             log.debug('CHANNEL_PINS_UPDATE referencing an unknown channel ID: %s. Discarding.', channel_id)
             return
 
         last_pin = utils.parse_time(data['last_pin_timestamp']) if data['last_pin_timestamp'] else None
 
-        try:
-            # I have not imported discord.abc in this file
-            # the isinstance check is also 2x slower than just checking this attribute
-            # so we're just gonna check it since it's easier and faster and lazier
-            channel.guild
-        except AttributeError:
+        if guild is None:
             self.dispatch('private_channel_pins_update', channel, last_pin)
         else:
             self.dispatch('guild_channel_pins_update', channel, last_pin)