Add raceN / firstCompletedOf recipe

[BalmungSan]

Motivated by a recent discussion on Discord.

Since this method can have many variations and is not very common, I guess a recipe is the best place to add it.

[ChristopherDavenport]

Should stop using this type generally and prefer the use of Outcome. This ends in a lot of weird deadlocks if folks use this without considering cancelation. So generally I say outcome or not at all.

[BalmungSan]

Well, the example mentions that the hypothetical user only cares about either success or error.
We may add a third example using Outcome or just being more explicit in the final note that another option is to consider cancelation (I prefer this one).

[armanbilge]

Can I throw one more implementation into the ring?

def raceN[A](ios: List[IO[A]]): IO[A] = ios match {
  case Nil => IO.raiseError[A](new NoSuchElementException)
  case head :: tail => tail.foldLeft(head)(_.race(_).map(_.merge))
}

Edit: fixed 😇

[BalmungSan]

I think this was OP's original implementation (or something similar)
It really doesn't feel natural, at least to me.

[djspiewak]

Playing more golf:

Suggested change

	Deferred[IO, A].flatMap { d =>
	ios.parTraverse_(io => io.flatMap(d.complete)).background.surround(d.get)
	}
	IO.deferred[Outcome[IO, Throwable, A]] flatMap { d =>
	ios.traverse(io => io.guaranteeCase(d.complete(_).void).background)
	.surround(d.get)
	.flatMap(_.embedNever)
	}

Though maybe I'm putting the suggestion in the wrong spot, since this isn't in any way the simplest implementation.