Add map method to NatTRel

Signature:

    trait NatTRel[L1 <: HList, F1[_], L2 <: HList, F2[_]] {
      def map(nt: F1 ~> F2, fa: L1): L2
    }

Allows mapping over HLists which have a consistent type constructor
structure with a natural transformation. For example, we can map from:

    Option[Int] :: Option[Boolean] :: Option[String] :: Option[Nothing] :: HNil

To:

    List[Int] :: List[Boolean] :: List[String] :: List[Nothing] :: HNil

Also delete problematic cases from NatTRel where Const occured on left
hand side. Doesn't seem to make sense when calculating the right hand
side's type.

Code reviewed by @milessabin