Async functions and await

Calling an async fn creates a lazy Future<T>. await starts it if needed, suspends the current task, and produces its T; only async functions may await.

async fn main() is the one entry whose future the launcher drives automatically. There is no synchronous helper that blocks on a future.

async fn answer() -> int {
  await sleep(5)
  42
}

async fn main() {
  println(await answer())
}