Consider adding an InvariantMonad

I came across the need for one recently and I think they can be fairly useful sometimes.
I've sketched out something based on @mpilquist's article

import cats._
import cats.implicits._

trait InvariantFlatMap[F[_]] extends InvariantSemigroupal[F] {

  def iflatMap[A, B](fa: F[A])(f: A => F[B])(g: B => A): F[B]
  
  override def product[A, B](fa: F[A], fb: F[B]): F[(A, B)] =
    iflatMap[A, (A, B)](fa)(a => imap[B, (A, B)](fb)(b => (a, b))(ab => ab._2))(ab => ab._1)
}

trait InvariantMonad[F[_]] extends InvariantMonoidal[F] with InvariantFlatMap[F] {
  def flatten[A](ffa: F[F[A]]): F[A] = iflatMap(ffa)(identity)(a => point(a))
  
  override def imap[A, B](fa: F[A])(f: A => B)(g: B => A): F[B] =
    iflatMap(fa)(a => point(f(a)))(g)
}

I think all of our current instances of InvariantMonoidal can also be InvariantMonads. I think it comes up enough to be useful in cats-core, but one could also envision this belonging into something like the proposed cats-more.

Start by reviewing the existing InvariantMonoidal instances and the proposed InvariantFlatMap and InvariantMonad definitions in the issue. Check the cats-core typeclass hierarchy and current instance constraints before deciding whether the abstraction belongs there or in cats-more. Done means the API design and placement have been agreed and the relevant instances are covered.