Bounded task channels

Channel<T> passes typed values between tasks. send waits while a bounded buffer is full, and recv waits while it is empty. Capacity zero hands each value directly from a sender to a receiver.

Closing preserves buffered values, then recv returns none. Sending after close is a runtime error.

async fn produce(channel: Channel<int>) {
  await channel.send(1)
  await channel.send(2)
  channel.close()
}

async fn main() {
  let channel = std.channel.new<int>(2)
  spawn(produce(channel))
  let mut total = 0
  while let some(value) = await channel.recv() { total += value }
  println(total)
}