array_unshift: Add fast optimized case for packed arrays

[ndossche]

Packed arrays are likely common in this case, as with array_shift which already has a similar optimization.

For the following benchmark:

<?php

for ($i = 0; $i < 10000000; $i++) {
    $a = [0, 1, 2, 3, 4, 5];
    array_unshift($a, -3, -2, -1);
}

On an i7-4790:

Benchmark 1: ./sapi/cli/php x.php
  Time (mean ± σ):     753.8 ms ±  23.8 ms    [User: 749.8 ms, System: 2.1 ms]
  Range (min … max):   734.3 ms … 818.6 ms    10 runs

Benchmark 2: ./sapi/cli/php_old x.php
  Time (mean ± σ):     972.5 ms ±   5.0 ms    [User: 968.8 ms, System: 1.4 ms]
  Range (min … max):   967.8 ms … 984.3 ms    10 runs

Summary
  ./sapi/cli/php x.php  ran
    1.29 ± 0.04 times faster than ./sapi/cli/php_old x.php

On an i7-1185G7:

Benchmark 1: ./sapi/cli/php x.php
  Time (mean ± σ):     507.9 ms ±   6.7 ms    [User: 504.7 ms, System: 2.6 ms]
  Range (min … max):   498.3 ms … 519.3 ms    10 runs
 
Benchmark 2: ./sapi/cli/php_old x.php
  Time (mean ± σ):     627.1 ms ±   7.2 ms    [User: 624.0 ms, System: 2.3 ms]
  Range (min … max):   616.6 ms … 640.2 ms    10 runs
 
Summary
  ./sapi/cli/php x.php ran
    1.23 ± 0.02 times faster than ./sapi/cli/php_old x.php

[iluuu1994]

Looks correct