Spawn and join tasks

spawn(future) starts cooperative work. join((a, b, ...)) starts every tuple child and returns results in source order, even when completion order differs. join_all does the same for a homogeneous array.

Failure or parent cancellation requests cancellation of unfinished siblings and drains their cleanup before the group settles.

async fn later(text: string, delay: int) -> string {
  await sleep(delay)
  text
}

async fn main() {
  let results = await join((later("slow", 20), later("fast", 5)))
  println("${results.0},${results.1}")
}