Optimize array_diff() for long values

[mehmetcansahin]

array_diff() currently builds its exclusion set from string representations, causing avoidable conversions for large integer-only inputs. This adds a collision-safe fast path using a bitset for compact integer ranges; mixed or sparse inputs continue through the existing path.

$a = range(0, 200000);
$b = range(100000, 300000);

$start = microtime(true);
for ($i = 0; $i < 10; $i++) {
    array_diff($a, $b);
}
printf("%.3fs\n", microtime(true) - $start);

Result: ~0.12s before, ~0.015s after (approximately 8x faster). Unlike the earlier integer-keyed HashTable approach, this retains linear worst-case behavior.

Tests cover integer limits, duplicates, key and reference preservation, and fallback behavior.