Singleflight Async

Singleflight in async style.

Example

```rust use singleflight_async::SingleFlight;

[tokio::main]

async fn main() { let group = SingleFlight::new(); let mut futures = Vec::new(); for _ in 0..10 { futures.push(group.work("key", || async { println!("will sleep to simulate async task"); tokio::time::sleep(std::time::Duration::from_millis(100)).await; println!("real task done"); "my-result" })); }

for fut in futures.into_iter() {
    assert_eq!(fut.await, "my-result");
    println!("task finished");
}

} ```

The output will be like: will sleep to simulate async task real task done task finished task finished task finished task finished task finished task finished task finished task finished task finished task finished