instances for other Integral indexes?

[aryairani]

We have some Word64-indexed types, and having to scatter fromIntegral all over to compensate outweighs the convenience of using like ifor_ f xs over for_ f (xs `zip` [0..]) in a smaller number spots.

WDYT about replacing instance FunctorWithIndex Int [] with instance Integral i => FunctorWithIndex i [], and so on?

[phadej]

instance Integral i => FunctorWithIndex i [], won't satisfy functional dependency and wreak havoc the type inference.