Fix long-standing issue with user updates not dispatching properly.

This fix is long coming. For a long time due to the addition of a
global user cache, the on_member_update event would only have the
updated user in the very first dispatch due to a quirk in the reference
only being updated once.

In order to fix this issue two things had to change:

1. There had to be a new event, `on_user_update` to complement
   the equivalent member event.
2. Unnecessary copies of User had to be removed to compensate for the
   performance hit from the diffing.

While doing these two fixes I also re-evaluated some more unnecessary
copies done during the PRESENCE_UPDATE to add member case while
fetch_offline_members=False is set or due to chunking issues. The
number of copies was brought down from 2 to 1, discounting the original
Member creation. Unsure on the benefits of this one, however.

N.B: this doesn't change the pre-existing behaviour of on_member_update

discord/state.py

@@ -461,11 +461,14 @@ class ConnectionState:
                 # skip these useless cases.
                 return
 
-            member = Member(guild=guild, data=data, state=self)
+            member, old_member = Member._from_presence_update(guild=guild, data=data, state=self)
             guild._add_member(member)
+        else:
+            old_member = Member._copy(member)
+            user_update = member._presence_update(data=data, user=user)
+            if user_update:
+                self.dispatch('user_update', user_update[0], user_update[1])
 
-        old_member = Member._copy(member)
-        member._presence_update(data=data, user=user)
         self.dispatch('member_update', old_member, member)
 
     def parse_user_update(self, data):