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/member.py

@@ -189,6 +189,17 @@ class Member(discord.abc.Messageable, _BaseUser):
         }
         return cls(data=data, guild=message.guild, state=message._state)
 
+    @classmethod
+    def _from_presence_update(cls, *, data, guild, state):
+        clone = cls(data=data, guild=guild, state=state)
+        to_return = cls(data=data, guild=guild, state=state)
+        to_return._client_status = {
+            key: value
+            for key, value in data.get('client_status', {}).items()
+        }
+        to_return._client_status[None] = data['status']
+        return to_return, clone
+
     @classmethod
     def _copy(cls, member):
         self = cls.__new__(cls) # to bypass __init__
@@ -200,7 +211,10 @@ class Member(discord.abc.Messageable, _BaseUser):
         self.nick = member.nick
         self.activities = member.activities
         self._state = member._state
-        self._user = User._copy(member._user)
+
+        # Reference will not be copied unless necessary by PRESENCE_UPDATE
+        # See below
+        self._user = member._user
         return self
 
     async def _get_channel(self):
@@ -230,9 +244,15 @@ class Member(discord.abc.Messageable, _BaseUser):
 
         if len(user) > 1:
             u = self._user
-            u.name = user.get('username', u.name)
-            u.avatar = user.get('avatar', u.avatar)
-            u.discriminator = user.get('discriminator', u.discriminator)
+            original = (u.name, u.avatar, u.discriminator)
+            # These keys seem to always be available
+            modified = (user['username'], user['avatar'], user['discriminator'])
+            if original != modified:
+                to_return = User._copy(self._user)
+                u.name, u.avatar, u.discriminator = modified
+                # Signal to dispatch on_user_update
+                return to_return, u
+        return False
 
     @property
     def status(self):