Adding `$yield` support to iterators

This allows the developer to use/specify a custom mapping function when iterating through a collection:

```scss
$iterator: iterator('abcde', $yield: 'to-upper-case');
$value: iterator-next($iterator);
// (done: false, value: 'A', collection: 'abcde', position: 1)
```

The default function is the `identity` function, which simply returns the exact same value.