round four: no variance in ListT
round five: no variance in IndexedContsT
round 6: OH GOD SO MANY INTERTWINED TRANSFORMERS
round 7: Kleisli, and while I'm at it, nuke UnapplyCo
round 8: Free, Zap, Co{density,free,product} and IO

Note: IO is not covariant anymore. It could still be, but that would
introduce quite some additional type parameters on its methods.