Proposal: Add `restriction` to Data.Map and Data.IntMap

[Henning Thielemann]

On Thu, 14 Jul 2016, David Feuer wrote:

> Cale Gibbard proposes the following:
>
> Data.IntMap.restriction :: IntSet -> IntMap a -> IntMap a
> Data.Map.restriction :: Ord k => Set k -> Map k a -> Map k a

I already used such functions and proposed them as intersectionSet, 
differenceSet and partitionSet:
    https://github.com/haskell/containers/issues/158