[php-src] PR #23947: Speed up zend_hash_iterator_add() when iterators are nested

Pull Request: https://github.com/php/php-src/pull/23947 Author: nicolas-grekas zend_hash_iterator_add() looks for a free slot in EG(ht_iterators) starting from slot 0 every time, so a foreach by reference, a foreach over object properties, an ArrayIterator or array_walk() pays one probe per iterator that's already alive. This adds EG(ht_iterators_free_hint): all slots below it are in use, zend_hash_iterator_add() starts looking from there and zend_hash_iterator_del() lowers it. The returned slot is still the lowest free one, so slot indexes and EG(ht_iterators_used) don't change. Found while profiling #23945: a recursive walk of 17.6k Twig nodes, each iterated through new ArrayIterator($this->nodes), spends 2.4M instructions in zend_hash_iterator_add()/del(). That's 1.5M with this patch, -1.2% on the whole walk (callgrind). Instructions per foreach ($a as &$v) {} over a 1-element array, nested in d-1 other loops by reference: 364 -> 365 at d=1, 389 -> 370 at d=5, 464 -> 369 at d=20. Same savings per loop for ArrayIterator, foreach over properties and array_walk().