Extend DepFnN for some HList ops

For the sake of consistency with the majority of the other HList Ops
type classes
Merge pull request #167 from ceedubs/extend-depfn

Extend DepFnN for some HList ops

This looks like a totally unobjectionable change to me. Hope I'm not overstepping my authority (obligatory: mwhaahah) in merging this.
Added head & tail to Coproduct