They're all syntax changes and renames. I've checked twice to make sure
I didn't miss any listeners or classes.
For future reference:
- Do `from discord.ext.commands import Cog` in all new cogs.
- Subclass the new class to Cog.
- New listeners should get the `@Cog.listener()` decorator.
Conveniently, there weren't any pre-execution methods that have been
renamed.
