[PoC] support sorting packed arrays without converting to hashed arrays first

[ndossche]

This is a proof-of-concept.

Demo speedup on i7-4790:

<?php

$x = range(20, 0);
for ($i = 0; $i < 500000; $i++)
    sort($x);
var_dump($x);

Benchmark 1: ./sapi/cli/php x.php 
  Time (mean ± σ):     123.2 ms ±   3.0 ms    [User: 119.5 ms, System: 2.6 ms]
  Range (min … max):   119.2 ms … 134.1 ms    24 runs
 
Benchmark 2: ./sapi/cli/php_old x.php
  Time (mean ± σ):     176.8 ms ±   2.5 ms    [User: 173.9 ms, System: 2.1 ms]
  Range (min … max):   174.3 ms … 183.2 ms    17 runs
 
Summary
  ./sapi/cli/php x.php  ran
    1.44 ± 0.04 times faster than ./sapi/cli/php_old x.php