IntMap.differenceKeysSet for removing an IntSet of keys

Currently, IntMap.difference ma mb removes all the keys in mb from ma, where the elements of the two IntMaps can be of different types; the elements of mb are not used.

There is no efficient way to remove an IntSet of keys, however. These patches adds the IntMap.differenceKeysSet function—essentially a copy/paste of difference—that satisfies the following property:

prop_DiffKeysSet :: Map Int -> Map () -> Bool
prop_DiffKeysSet t1 t2
  = difference t1 t2 == differenceKeysSet t1 (keysSet t2)

Trac metadata

Trac field	Value
Version	7.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries (other)
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