Re-export lookup for Map

[Sorokin-Anton]

Problem

We're re-exporting Map from containers and HashMap from unordered-containers, but there is no way to efficiently lookup a value by key without adding new dependencies, e.g. we're not reexporting Map.lookup and Map.!?

Note that we already have ToPairs t typeclass that have Key t and Val t type synonyms, and there are instances for Map and HashMap

Possible solutions

• export lookupMap for Map and lookupHashMap fro HashMap (I don't like it, since I like the current classy-approach).
• Add lookup and !? to ToPairs t class with Prelude.lookup . toPairs default implementation. Use proper functions for Map and HashMap instances, adding necessary (Ord/Hashable k, Eq v) constraints to those instances. I like this approach, but one can accidentally use lookup for [(a,b)], which is slow enough if compare to Map/HashMap.
• Add new class ToPairs t => Lookup t where lookup :: t -> Key t -> Maybe (Val t), make instances for Map and HashMap.