Proposal: composition util for Maps

[Alexandre Esteves]

In https://github.com/haskell/containers/issues/647 I proposed the
following util:

composition :: Ord b => Map b c -> Map a b -> Map a c
composition bc ab = flip mapMaybe ab $ flip lookup bc

Which has O(|ab| * log |bc|) performance. It's not particularly hard to
write, but it does feel like a primitive-ish operation, and it's not
obvious to me whether there's a faster implementation.

Other name suggestions
- (.)
- chain
- compose
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