[HypeMC]

Follow-up to #234.

The underlying Heap iterator bugs predated #234, but returning InternalIterator exposed them through direct current(), key(), and rewind() calls. Ordinary foreach checks valid() before reading values or keys and creates a fresh iterator for each loop, which hid these cases.

This fixes value ownership and rewinding, and returns null for empty or exhausted iterators.

Repeated current() calls no longer access freed objects:

$heap = new Ds\Heap([(object) ['value' => 1]]);
$it = $heap->getIterator();
$heap->clear();
echo json_encode($it->current()); // {"value":1}
echo json_encode($it->current()); // {"value":1}; previously could crash

Rewinding restarts the same iterator:

$it = (new Ds\Heap([1, 3, 2]))->getIterator();
iterator_to_array($it); // [3, 2, 1]
iterator_to_array($it); // [3, 2, 1]; previously []

Empty or exhausted iterators now return null from current() and key(). Calling next() or rewind() on the same iterator from its comparator throws RuntimeException to prevent use-after-free.