Deprecate and remove ifA

[chuwy]

I recently ran into a very dangerous bug thinking ifM and ifA have the same semantics. It can be repdocued with:

def test[F[_]: Sync] =
  Ref.of[F, Map[Boolean, Unit]](Map.empty).flatMap { ref =>
    Sync[F].pure(true).ifA(
      ref.update { c => c ++ Map(true -> ())},
      ref.update { c => c ++ Map(false -> ())}
    ) *> ref.get
  }

@ test[IO].unsafeRunSync()
res6: Map[Boolean, Unit] = Map(true -> (), false -> ())

I'd argue that we never ever ever want to use this just with IO. But it's not much better (at first glance) with other data types:

@ true.some.ifA(true.some, false.some)
res0: Option[Boolean] = Some(true)

@ true.some.ifA(true.some, none)
res1: Option[Boolean] = None

@ true.some.ifA(none, false.some)
res2: Option[Boolean] = None

I do understand now that this is an intended behavior of executing all three effects and we simply cannot do what I wanted to do with Applicative (we can though with selective functors, https://github.com/kowainik/validation-selective), but I still think the potential risk overweight the usefulness -also just grepp'ed few popular projects (doobie, http4s, fs2) and nobody uses it it seems.

Assuming the decision has been made I can create a PR deprecating it.