Invariant super class of Alternative and Decidable

This ties into #1935, but also #2400 and #1932.

I think we could do a lot better with the Alternative hierarchy and the Decidable PR shows, it might need some binary breaking changes.

Ideally we could replicate the *Monoidal hierarchy, but using sum types instead. Then at the end we'd have InvariantSemiringal and have Alternative and Decidable extend from that.

Some code I came up with (names are not final by any means):

// Invariant hierarchy

@typeclass
trait InvariantAddSemigroupal[F[_]] {
  def invariant: Invariant[F]
  def sum[A, B](fa: F[A], fb: F[B]): F[Either[A, B]]
}

@typeclass
trait InvariantAddMonoidal[F[_]] extends InvariantAddSemigroupal[F] {
  def empty[A]: F[A]
}

@typeclass 
trait InvariantSemiringal[F[_]] extends InvariantAddMonoidal[F] with InvariantMonoidal[F]

// Covariant hierarchy

@typeclass
trait SemigroupK[F[_]] extends InvariantAddSemigroupal[F] {
  def combineK[A](x: F[A], y: F[A]): F[A]
  def functor: Functor[F]

  override def invariant: Invariant[F] = functor
  override def sum[A, B](fa: F[A], fb: F[B]): F[Either[A, B]] =
     combineK(functor.map(fa)(Left(_)), functor.map(fb)(Right(_)))
}

@typeclass
trait MonoidK[F[_]] extends SemigroupK[F] with InvariantAddMonoidal[F]

@typeclass
trait Alternative[F] extends MonoidK[F] with InvariantSemiringal[F]

// Contravariant hierarchy

@typeclass
trait ContravariantAddSemigroupal[F[_]] extends InvariantAddSemigroupal[F] {
  def contravariant: Contravariant[F]

  override def invariant: Invariant[F] = contravariant
}

@typeclass
trait ContravariantAddMonoidal[F[_]] extends ContravariantAddSemigroupal[F] with InvariantAddMonoidal[F] 

@typeclass
trait Decidable[F] extends ContravariantAddMonoidal[F] with InvariantSemiringal[F]

I've also written about this quite a bit in this blog post: https://typelevel.org/blog/2018/11/02/semirings.html

I made a funky looking diagram real quick:

As to why this is useful, I think this offers a really great API for codec combinators when working with e.g. circe, scodec or recently skunk.

Start by reading the existing Alternative and Decidable hierarchies, then review related issues #1935, #2400, and #1932 and the linked semirings blog post. Done would require an agreed hierarchy design, including names and binary-compatibility decisions; the issue does not identify files or tests.