Missing TupleK Semigroupal

[johnynek]

I don't see an implementation (although there is ContravariantMonoidal, which is more constrained).

I'll plan to send a PR, but if anyone sees a reason why this can't exist, please feel free to let me know.

[johnynek]

this seems to work just fine, and I think is lawful:

  implicit def semigroupalTuple2K[F[_]: Semigroupal, G[_]: Semigroupal]: Semigroupal[Tuple2K[F, G, *]] =
    new Semigroupal[Tuple2K[F, G, *]] {
      def product[A, B](left: Tuple2K[F, G, A], right: Tuple2K[F, G, B]): Tuple2K[F, G, (A, B)] =
        Tuple2K(
          Semigroupal[F].product(left.first, right.first),
          Semigroupal[G].product(left.second, right.second))
    }

[rmehri01]

Hi, I can try adding this.