(I'm going to make this change:)

We don't really need expand here: expand is only worthwhile if we're doing some kind of arithmetic and need it to be unboxed (e.g. a + b). 

This is what I'm going to do instead:
```scala
implicit def dvReverse[T:ClassTag]: Impl[DenseVector[T], DenseVector[T]] =
    new Impl[DenseVector[T], DenseVector[T]] {
      def apply(dv: DenseVector[T]): DenseVector[T] = DenseVector( dv.toArray.reverse )
    }
```