Potential StackOverflowError in mapK of RebalanceCallback

[nikitapecasa]

@t3hnar says

I believe this may cause stackoverflow for huge enough data structure, however I don't think you should fix this.

def mapK[G[_]](fg: F ~> G): RebalanceCallback[G, A] = {
      self match {
        case Pure(a)          => Pure(a)
        case Bind(source, f)  => Bind(source.mapK(fg), f andThen (_.mapK(fg)))

[nikitapecasa]

Fixed with

// lazy source is needed to avoid StackOverflowError in mapK implementation
   final case class Bind[F[_], S, +A](source: () => RebalanceCallback[F, S], fs: S => RebalanceCallback[F, A])
       extends RebalanceCallback[F, A]

in scope of #135
and there are tests proving that it's stack safe now