Refactor Guild to support type hints

This patch also does the following:

* Sets some parameters to be positional only
* Changes Guild.edit to use the MISSING sentinel
* Changes the various create_channel methods to be type safe
* Changes many parameters from Optional[T] to use MISSING
* Changes Guild.create_role to use MISSING sentinel

This refactor is mostly partial but lays a decent foundation

discord/state.py

@@ -44,6 +44,7 @@ from .mentions import AllowedMentions
 from .partial_emoji import PartialEmoji
 from .message import Message
 from .channel import *
+from .channel import _channel_factory
 from .raw_models import *
 from .member import Member
 from .role import Role