Optimization for immediate operations in empty queues

For example, can we avoid a hop when the queue is empty and the operation is synchronous?

```swift
let queue = AsyncQueue()
await queue.perform { print("Hello") }
// <- Here "Hello" would be printed already.
```

Similarly, could we add support for immediate tasks ([SE-0472](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0472-task-start-synchronously-on-caller-context.md))?

```swift
let queue = AsyncQueue()
await queue.performImmediately { ... }
queue.addImmediateTask { ... }
```

The latter APIs are problematic, though, because the operation won't start immediately if the queue is not empty.