Implement iteration protocol in `TrainrunIterator`

[emersion]

See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols

This would allow for a more natural iteration.

Before:

while (iterator.hasNext()) {
  const pair = iterator.next();
  // …
}

After:

for (const pair of iterator) {
  // …
}

[aiAdrian]

We have to take care that we not run into side-effects. But I fully understand the issue and support this taks.