Optimize array_slice for packed/unpacked arrays with large offsets

[TysonAndre]

If the offset is 100000, and there are no gaps in the packed array,
then advance the pointer once by 100000(1+99999),
instead of looping and skipping 100000 times.

This was validated with this test script. Unit tests continue to pass locally:

<?php
$example = range('x', 1000000);
$start = microtime(true);
$output = '';
for ($i = 0; $i < 10; $i++) {
    $value = array_slice($example, 999900 + $i, 5);
    $output .= json_encode($value) . "\n";
}
// With this patch:    0.000010 (best run)
// Without this patch: 0.026424
printf("Elapsed time: %.6f\n", microtime(true) - $start);
echo $output;

[TysonAndre]

Also, I should probably make the same optimization for $preserve_keys=true - It'd be inconsistent to have one be thousands of times slower than the other.

[TysonAndre]

The review comments have been addressed. The build failure was due to a mac OS job encountering a networking issue.