Give futures::executor::ThreadPool a possibility to wait for completion

[TobiasJacob]

Consider the following example for the threadpool, which is close to the example from the documentation:

https://docs.rs/futures/0.3.15/futures/executor/struct.ThreadPool.html

use std::thread::sleep;
use std::time::Duration;

use futures::FutureExt;
use futures::executor::ThreadPool;

async fn cpu_intensive(i: u64) {
    let mut res: u64 = 0;
    for j in 0..23 {
        for k in 0..256 {
            res += j + i + k;
            res %= 91349;
        }
        // Imagine that here an async API call is required, so we want another future to continue while this future is waiting for the API result 
    }
    println!("{}: {}", i, res)
}

fn main() {
    let pool = ThreadPool::new().unwrap();
    for i in 0..1000 {
        let fut = cpu_intensive(i);
        pool.spawn_ok(fut)
    }
    // NEEDED: A way to wait for the thread pool to run until completion
    sleep(Duration::from_secs(2));
}

It would be great if there was a way to block main until all futures of the ThreadPool are completed.