MultiProgress: should prune leading progress bars that have become `DoneVisible` or `DoneHidden`

Consider this small example program:

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

use indicatif::{MultiProgress, ProgressBar, ProgressFinish};

fn main() {
    let m = MultiProgress::new();
    let height = console::Term::stdout().size().0;

    for i in 0..(height + 1) {
        let p1 = m.add(
            ProgressBar::new_spinner()
                .with_message(format!("p{}", i))
                .with_finish(ProgressFinish::AndLeave),
        );
        p1.enable_steady_tick(Duration::from_millis(50));
        thread::sleep(Duration::from_millis(200));
    }
}

It creates one more spinner that there are lines in the terminal. Here is a video of it running:

https://user-images.githubusercontent.com/40474653/166507072-efdbaf25-82e5-4f25-91ac-e6ab51db2314.mp4

Since we never call MultiProgress::remove on ProgressBars that are dropped/finished (see #419), bars are always redrawn even when they don't need to be. When the number of bars (n) becomes greater than terminal lines (m), on each draw MultiProgress will clear the last m lines but write n lines, leaving m - n lines in the scrollback history.

I propose the following algorithm. On each MultiProgress:draw, iterate over the bars in order (according to MultiState::ordering) and take_while(|bar| bar.is_finished()). This set of leading progress bars is then pruned by calling MultiProgress:remove on each one. This ensures we are not re-drawing bars that don't need to be redrawn and that we are some reclaiming memory.

The wrinkle is that MultiState cannot currently get at the ProgressState to check is_finished(), so we will need to restructure some code.

console-rs / indicatif

MultiProgress: should prune leading progress bars that have become `DoneVisible` or `DoneHidden` #426