I assume that `SetK` doesn't have a functor instan...

# arrow

[Egor Trutenko]

I assume that SetK doesn't have a functor instance because set from Haskell doesn't as well, and Haskell was an inspiration for Arrow (right?). But there are reasons for such problems in Haskell, whereas in Kotlin we could map over sets quite good. @raulraja perhaps Arrow might introduce functor for SetK?