MultiBar only works with threads

[njaard]

The only way to display multiple progress bars is to use threads, this requirement isn't documented, and when you use single threads, you get silent failures.

[fosskers]

I'm currently trying to get it working with rayon, we'll see if I'm successful.

[fosskers]

The following prints nothing, not even the DONE! messages:

pub(crate) fn bar_test() {
    let mb = Arc::new(MultiBar::new());
    std::thread::spawn({
        let mb = Arc::clone(&mb);
        move || {
            mb.listen();
        }
    });

    (0..30).into_par_iter().for_each(|n| {
        let mut bar = mb.create_bar(100);
        bar.message(&format!("{} ", n));

        for _ in 0..100 {
            bar.inc();
            std::thread::sleep(Duration::from_millis(50));
        }

        bar.finish_print("DONE!");
    })
}

The process does complete and exit, which means the loops are occuring, but the bars seem to be lost.

[fosskers]

If I add a second mb.listen() after the for_each loop, the DONEs all get printed in bulk after the entire thing exits. PR #92 mentions that bars can be created dynamically after the listen has been initiated, but that doesn't seem to work in this case.