Add leftMap method to data types that have Bifunctor instances
Set is not a Functor
Use getOrElseUpdate to simplify implementation for mutable maps
Merge pull request #283
Merge pull request #276 from larsrh/topic/set-functor

Set is not a Functor
Merge pull request #285 from mpilquist/leftMap

Add leftMap method to data types that have Bifunctor instances
Add contravariant laws; test them on Function1.
Contravariant instance for Kleisli.
Merge pull request #287 from S11001001/topic/contravariant

Contravariant laws; instance for Kleisli.