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

[Ross Paterson]

On Thu, Jul 14, 2016 at 12:45:27AM -0400, 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
> 
> In each case, the map is filtered to contain only the keys that are
> also found in the set. This can be implemented efficiently using a
> slightly stripped-down version of Data.Map.intersection.

One might also want the counterpart of Data.Map.difference for similar reasons.