How could monadIOLaws ever be used?

To have a law-abiding `MonadIO` means that your monad must be at least as powerful as `IO` - ie, be a transformer over `IO`. Such a monad won't have a meaningful `Eq` instance, because `IO` has no `Eq` instance. Can `monadIOLaws` actually ever be used?

Start by examining the `monadIOLaws` definition and its relationship to `MonadIO`, `IO`, and the required `Eq` instance. Determine whether any valid transformer-over-`IO` type can satisfy the law, and document whether the law should remain available or be redesigned.

Written by the indexing model from the issue text.