Change certain sequences to use a special proxy type instead of list

This is to speed up cases where someone is just querying the length
of the underlying sequence. If anything else is done to the sequence
then it is copied from the original iterator.

This change should be mostly transparent.

docs/migrating.rst

@@ -864,6 +864,25 @@ The return type of the following methods has been changed to an :term:`asynchron
 The ``NoMoreItems`` exception was removed as calling :func:`anext` or :meth:`~object.__anext__` on an
 :term:`asynchronous iterator` will now raise :class:`StopAsyncIteration`.
 
+Changing certain lists to be lazy sequences instead
+-----------------------------------------------------
+
+In order to improve performance when calculating the length of certain lists, certain attributes were changed to return a sequence rather than a :class:`list`.
+
+A sequence is similar to a :class:`list` except it is read-only. In order to get a list again you can call :class:`list` on the resulting sequence.
+
+The following properties were changed to return a sequence instead of a list:
+
+- :attr:`Client.guilds`
+- :attr:`Client.emojis`
+- :attr:`Client.private_channels`
+- :attr:`Guild.roles`
+- :attr:`Guild.channels`
+- :attr:`Guild.members`
+
+This change should be transparent, unless you are modifying the sequence by doing things such as ``list.append``.
+
+
 Removal of ``Embed.Empty``
 ---------------------------