Add doc-tests

[kpreid]

The functions should have code samples and they should be tested.

[kpreid]

In order to test, they will need some kind of construction of a YieldProgress with readable output. Should they visibly have some kind of let (progress, cell) = yield_progress::testing::ProgressCell::new(); or should they hide that as a distraction?

I do think that we want testing utilities so that code which consumes YieldProgress can be tested easily.

[kpreid]

I tried to sketch a test with no additional dependencies...

use std::sync;
use std::future::Future;

fn labeled_progress(progress: YieldProgress) {
    progress.set_label("one");
    progress.progress(0.0).await;
    progress.set_label("two");
    progress.progress(0.5).await;
    progress.finish().await;
}

// Create a `YieldProgress` we can look at.
let current_label: sync::Arc<sync::Mutex<String>>> = Default::default();
let progress = yield_progress::Builder::new()
    .progress_using({
        let current_label = current_label.clone();
        move |info| *current_label.lock().unwrap() = info.label_str().to_owned()
    });

let task = labeled_progress(progress);
tokio::pin!(task);
task.poll(...)

Doesn't seem very nice to read and we still need a noop waker to finish implementing it.