for (const iter of iters) {
    for (const val of iter) {
      yield val;
    }
  }
Combines multiple Iterables into a single Iterable.
Traverses the input Iterables in the order provided and maintains the order
of their elements.

Example:
```
const r = Array.from(concatIterables(['a', 'b'], ['c'], ['d', 'e', 'f']));
r == ['a', 'b', 'c', 'd', 'e', 'f'];
```