Select readiness and cancel

select returns the index of the first future to finish. cancel requests cooperative cancellation, and is_cancelled lets long-running work decide when to stop.

Cancellation is a request, so a task can finish its cleanup before it settles.

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

async fn main() {
  let slow = spawn(later(10, 20))
  let fast = spawn(later(20, 5))
  println(await select([slow, fast]))
  cancel(slow)
}