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

[David Feuer]

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.

David Feuer