Cancel all operations

Can we add a `cancelAllOperations()` method?

Currently, this conflicts with the fact that the `perform` methods prefer running in the current task:

```swift
// AsyncQueues 0.3.0

let queue = AsyncQueue()
await queue.perform {
// In the same task as the caller
}
```

For `cancelAllOperations()` to work, we'd need to run those operations in a distinct task:

```swift
// With hypothetic support for cancelling all operations

let queue = AsyncQueue()
await queue.perform {
// Must be in a distinct task than the caller
// so that we can cancel this operation.
}
```

This would be a breaking change:

```swift
let queue = AsyncQueue()

await queue.perform {
// Would not cancel the same task
withUnsafeCurrentTask { currentTask in
currentTask?.cancel()
}
}
```

cc @mattmassicotte - in case you'd have hindsights about the differences between running in the current task vs. a new top-level Task created from the current task, and why one could prefer one or the other.