`vector-space-points` makes sense if you don't mind adding the `lens` dependency.  I don't think it's possible to define an `Iso` without depending on `lens`, because of `Profunctor`.  I think `Diagrams.Core.Points` is a good place, also.  Should we write it as `Iso' (Point v) v` or the more general `Iso' p (Diff p)`?