Add annotateSet

[milesfrain]

Wanted to make this suggestion isn't forgotten after #31 is merged.

annotateSet :: forall a b. (a -> b) -> Set a -> Map a b
annotateSet f = mapWithIndex (\k _ -> f k) <<< toMap

[darichey]

This would be great. For searchability, this is fromSet in Haskell's containers.