More set/map interactions

[treeowl]

I was thinking today that it might be nice to have things like

fromSetA :: Applicative f => (k -> f a) -> Set k -> f (Map k a)
fromSetWithIndexA :: Applicative f => (Int -> k -> f a) -> Set k -> f (Map k a)
fromSetWithIndex :: (Int -> k -> a) -> Set k -> Map k a
fromSetWithIndex f s = runIdentity (fromSetWithIndexA (coerce f) s)

These would let the user produce the values in an arbitrary Applicative, and the WithIndex version would let the passed function know where in the set it was working. For example,

counting :: Set k -> Set k Int
counting = fromSetWithIndex (\i _ -> i)

-- counting (fromList "abc") = fromList [('a', 0), ('b', 1), ('c', 2)]

I'd also really love to get something like mergeA for Set, and ideally something similar that would combine sets with maps. Having to convert a set to a map first seems pretty bad.

[meooow25]

I'm on board with adding fromSetA. See also: #815 (comment)

I'm not too sure about the WithIndex versions.