AsyncBencher::iter_batched and iter_batched_ref should allow for async setup

[shepmaster]

AsyncBencher::iter_batched and iter_batched_ref should allow for async setup

[vnermolaev]

Indeed, I have faced a similar and more extended version of this problem.

I came up with the following workaround for my particular case:

fn setup(rt: &runtime::Runtime) -> u64 {
    rt.block_on(async {
        // do async stuff
        tokio::time::sleep(Duration::from_millis(100)).await;
        1
    })
}

async fn test() -> u64 {
    1
}

pub fn criterion_benchmark(c: &mut Criterion) {
    let rt = runtime::Builder::new_multi_thread()
        .enable_all()
        .build()
        .unwrap();

    let setup = setup(&rt);

    c.bench_function("demo", |b| {
        b.to_async(&rt)
            .iter_batched(|| setup.clone(), |_| async {}, BatchSize::SmallInput);
    });
}