Add subscribeEventedEffOn

So there's probably a slightly more elegant way of defining this so you don't have to duplicate the entire of `subscribeEventedOnPrime` for `subscribeEventedEffOnPrime`, but as you can see the changes are fairly minor.
This `fnE` is what you're missing basically. You need to apply the handler "twice" when using Eff, as the first call is returning an `Eff`, and the second call (with no argument) is making the effect actually happen.