Keep int elements findable after removeFirst() on hash sets

[delacry]

HashElementStore keys each element by KeyHasher::hashSetKey(), which for an int is the int itself. removeFirst() came from AbstractElementStore and used array_shift(), which renumbers integer keys, so the remaining ints ended up under the wrong keys:

$set = mutableSetOf([5, 7, 9]);
$set->removeFirst();
$set->contains(7);        // before: false, after: true
$set->add(7)->toArray();  // before: [7, 9, 7], after: [7, 9]

The same happened on immutable sets, since ImmutableSet::removeFirst() calls the store on a clone. Sets of strings weren't affected because array_shift() keeps string keys.

HashElementStore now overrides removeFirst() and unsets the first key, as it already did for removeIf(), sort(), reverse() and shuffle(). ArrayIndexStore keeps using array_shift(), since lists need the renumbering.

Tests: removeFirst() on an int set, then contains() and add() (failed on all four set types before).

[codecov]

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