Use a dict instead of getattr for parsing events.

Probably not a significant difference but might as well use it here.
The basic idea is to cache the getattr calls instead of repeatedly
doing it (since they're around 105ns on my machine). The dictionary
lookup is about 41ns on my machine.

The next step in speeding up library code some more should be in
the parser bodies themselves but that's a problem to tackle another
day.

discord/shard.py

@@ -221,6 +221,7 @@ class AutoShardedClient(Client):
 
         ws.token = self.http.token
         ws._connection = self._connection
+        ws._discord_parsers = self._connection.parsers
         ws._dispatch = self.dispatch
         ws.gateway = gateway
         ws.shard_id = shard_id