Add Profunctor, Choice, Contravariant

[neongreen]

This would allow people to define lens-style Folds, Prisms and Isos without depending on lens.

[Roxxik]

Having profunctors in base would allow lens to use profunctor based lenses which have some advantages

see https://github.com/purescript-contrib/purescript-lens/issues/26

Edit: here is a nice discussion of the package <-> lens feature correspondence: https://github.com/ekmett/lens/issues/647