ArrowChoice case syntax works for Decidable

[tomjaguarpaw]

I was surprised to observe that ArrowChoice \case syntax[1] works[2] for Decidable even in the absence of any other arrow syntax. We can quite happily write

\case
    [] -> f -< ()
    [x] -> g -< x+1
    x1:x2:xs -> do
        h -< (x1, x2, xs)

to get something of type f [Int] when Decidable f, f :: f (), g :: f Int, h :: f (Int, Int, [Int]). This addresses a problem I observed a while ago about the incomposability of Decidable.

Perhaps I shouldn't surprised that this is possible. It's dual to how the the opposite half of arrow notation (Applicative and <-) supports ApplicativeDo.

[1] Which doesn't actually seem to be supported by arrow notation, sadly
[2] Or rather "would work"