For this benchmark:
```php
$length = 25;
for ($i=0;$i<1000;$i++)
  array_chunk(range(0, 10000), $length);
```

On an i7-4790, length=25 speeds up by 1.8x and length=1 by 1.27x.
On an i7-1185G7, length=25 speeds up by 1.08x and length=1 by 1.02x.

By preallocating it as a packed array, we save the initial
initialization overhead.
Gives a few extra percentage points improvement.