Add map, traverse etc.

[paf31]

cc @justinwoo @joneshf

I think it would be interesting to add these for Homogeneous records, once we merge Type.Row.Homogeneous.

[paulyoung]

This was merged and released purescript/purescript-typelevel-prelude#20.

[paulyoung]

FYI @natefaubion, since you and I have discussed having sequence for records.

[paf31]

We've discussed this before, and I think it belongs in a separate repo. The reason is that there is not just one sensible definition of map for records, and Homogeneous could be the basis of a newtype and an associated list of instances.