Support effect of boolean

[adrian-2414745]

Hello, today I tried using this lib, from the provided example:

 val p: PropF[IO] = 
      PropF.forAllF { (x: Int) =>
        IO(x).start.flatMap(_.join).map(res => assert(res == x))
      }

but it was not clear to me why the assert is needed there, actually in my case instead of the assert(res == x) I had a scalatest.Assertion and it kept asking me for an implicit IO[Assertion] => PropF[F] which i found very hard to do.

I solved it by using PropF.boolean but it took a while to discover that this is probably what i should use.

Now if I am not missing anything else, I think this would we much more intuitive:

PropF.forAllF( (a: Int) => IO(a == a))

with the help of

  implicit def effectOfBooleanToPropF[F[_]](
      fu: F[Boolean]
  )(implicit F: MonadError[F, Throwable]): PropF[F] = {
    val fb: F[PropF[F]] = F.map(fu) { a => PropF.boolean[F](a)}
    effectOfPropFToPropF(fb)
  }

then the user can map any F[A] to F[Boolean]
WDYT ?
If ok i can try a PR.

[mpilquist]

@adrian-salajan Apologies on the delay in response. I think supporting IO[Boolean] is a good idea. We currently support IO[Unit] so the original could have been .map(res => res shouldBe 42).void

[TonioGela]

A PR for this got merged, but I forgot to add the Closes #25 :
Also, this is a gentle request for a release, so that I can advocate this lib at work :D