Fix memory leak by holding on to Emoji references weakly.

The library had a memory leak in the case using the global emoji cache.
When the bot would leave the guild, the Emoji would maintain a strong
reference to the Guild keeping them alive along with the entire state
associated with it.

discord/state.py

@@ -70,8 +70,8 @@ class ConnectionState:
     def clear(self):
         self.user = None
         self._users = weakref.WeakValueDictionary()
+        self._emojis = weakref.WeakValueDictionary()
         self._calls = {}
-        self._emojis = {}
         self._guilds = {}
         self._voice_clients = {}
         self._private_channels = {}