Move Data.Functor.Contravariant into base

[Andrew Martin]

The typeclass Contravariant (from the contravariant package) is both
useful and fundamental. I would like to see this moved into base.
One additional motivating factor is that it would become possible 
for a DeriveContravariant extension to be written in a future GHC
release.  I'd love to hear other people's thoughts, even if it's as
simple as a yea or nay. Thanks.

-Andrew Martin