Remove covariance from FrameEffect

[JD557]

Removes the covariance requirement from FrameEffect.

This didn't make much sense anyway, as that parameter would be either ignored (on a stateless loop) or be be used as an input for the loop (on a stateless loop).

This also allows for some very primitive cats-effect interop with

// Not really needed, but it helps to be able to use all minart-pure helpers
implicit def asSyncIO[S, A](rio: RIO[S, A]): Kleisli[SyncIO, S, A] = Kleisli.ask[SyncIO, S].map(s => rio.run(s))

implicit val catsFrameEffect: FrameEffect[[S, A] =>> Kleisli[SyncIO, S, A]] = new FrameEffect {
  def contramap[A, AA, B](f: Kleisli[SyncIO, A, B], g: AA => A): Kleisli[SyncIO, AA, B] = f.local(g)
  def unsafeRun[A, B](f: Kleisli[SyncIO, A, B], subsystem: A): B = f.run(subsystem).unsafeRunSync()
}

Which, while not great (I'm still not sure how I can support async IO yet), it's already a nice step forward towards #18 .