array_chunk(): Fill packed chunks directly when keys are not preserved

[mehmetcansahin]

This PR makes array_chunk() faster when $preserve_keys is false, which is the default.
In this case, each chunk is always a list, so PHP can fill it directly instead of adding the values one by one.
This works for both packed arrays and arrays with string keys, including arrays with removed elements. Reference handling stays the same, and the $preserve_keys = true path is unchanged.
Benchmarks show a 2.1×–2.8× speedup.

Benchmarks

array_chunk($input, $size, false) on an Apple M1 using a release build (best of three runs).

input	size	repetitions	base	this PR	speedup
packed, n=100	10	200,000	144.5 ms	69.7 ms	2.1×
packed, n=1,000	10	20,000	139.9 ms	64.0 ms	2.2×
packed, n=1,000	100	20,000	105.3 ms	38.5 ms	2.7×
packed, n=100,000	100	200	110.7 ms	38.9 ms	2.8×
string keys, n=1,000	10	20,000	140.0 ms	64.1 ms	2.2×

Benchmark script

<?php
function bench(string $label, array $input, int $size, bool $preserve, int $reps): void {
    $best = PHP_FLOAT_MAX;
    for ($run = 0; $run < 3; $run++) {
        $t = hrtime(true);
        for ($r = 0; $r < $reps; $r++) {
            array_chunk($input, $size, $preserve);
        }
        $best = min($best, (hrtime(true) - $t) / 1e6);
    }
    printf("%-28s size=%-5d x%-7d best: %8.1f ms\n", $label, $size, $reps, $best);
}

$hash = [];
for ($i = 0; $i < 1000; $i++) { $hash["k$i"] = $i; }

bench('packed n=100',    range(0, 99),    10,  false, 200000);
bench('packed n=1000',   range(0, 999),   10,  false, 20000);
bench('packed n=1000',   range(0, 999),   100, false, 20000);
bench('packed n=100000', range(0, 99999), 100, false, 200);
bench('string keys n=1000', $hash,        10,  false, 20000);
bench('packed n=1000 preserve', range(0, 999), 10, true, 20000);