Return InternalIterator from collection getIterator() methods

[HypeMC]

See twigphp/Twig#3857.

When using Twig with ext-ds, filters such as last can result in an infinite loop.

The issue is caused by the following loop in CoreExtension:

while ($item instanceof \IteratorAggregate) {
    $item = $item->getIterator();
}

ext-ds collections implement IteratorAggregate, but their getIterator() currently returns another IteratorAggregate, causing code such as the above to keep unwrapping indefinitely.

PHP provides InternalIterator for exposing the internal iterator of internal classes. InternalIterator implements Iterator, not IteratorAggregate.

This PR changes ext-ds to return an InternalIterator from getIterator(), matching the intended behavior for internal iterators and avoiding the infinite loop.