Add 'partitionOn'

[chshersh]

Currently, slist has the following function:

partition :: forall a. (a -> Bool) -> Slist a -> (Slist a, Slist a)

But sometimes it's desirable to change the types of lists, and the function like below can be helpful:

partitionOn :: (a -> Either b c) -> Slist a -> (Slist b, Slist c)

The name can be different 🙂 But I wanted something like this for ordinary lists so many times...