`AsyncChannel` crash when cancelled with only one awaiting

[twittemb]

Hi,

AsyncChannel can crash in the following context:

• start an iteration in a Task
• cancel the task while the AsyncChannel is being awaited
• send a value in the channel

Here is a test that crashes:

func test_cancel_when_only_one_iterator_while_waiting() async throws {
  let channel = AsyncChannel<Int>()

  let task = Task {
    for await _ in channel {}
  }

  try await Task.sleep(nanoseconds: 1_000_000_000) // ensures the loop is waiting for a next value

  task.cancel() // sets the inner state of the channel to awaiting but with 0 nexts

  await channel.send(1) // tries to remove the first element of the awaiting state (which is empty) -> crash
}

linked to #143

[twittemb]

The PR has been merged and an issue has been created to track the potential unit tests #148