Optimize SplFixedArray::toArray()

[nielsdos]

We can use the optimized packed filling code instead of going through all the logic of the zend_hash update APIs.

For this script:

$test = new SplFixedArray(4);
$test[0] = 0;
$test[1] = 1;
$test[2] = 2;
$test[3] = 3;

for ($i = 0 ; $i< 5000000; $i++)
	$test->toArray();

On an i7-4790:

Benchmark 1: ./sapi/cli/php toarray.php
  Time (mean ± σ):     170.0 ms ±   1.8 ms    [User: 167.3 ms, System: 2.2 ms]
  Range (min … max):   166.9 ms … 173.0 ms    17 runs

Benchmark 2: ./sapi/cli/php_old toarray.php
  Time (mean ± σ):     215.7 ms ±   3.6 ms    [User: 211.9 ms, System: 3.0 ms]
  Range (min … max):   211.3 ms … 222.0 ms    13 runs

Summary
  ./sapi/cli/php toarray.php ran
    1.27 ± 0.02 times faster than ./sapi/cli/php_old toarray.php

On an i7-1185G7:

Benchmark 1: ./sapi/cli/php toarray.php
  Time (mean ± σ):     112.6 ms ±   1.4 ms    [User: 109.6 ms, System: 2.9 ms]
  Range (min … max):   111.1 ms … 116.4 ms    25 runs

Benchmark 2: ./sapi/cli/php_old toarray.php
  Time (mean ± σ):     145.3 ms ±   2.8 ms    [User: 141.8 ms, System: 3.4 ms]
  Range (min … max):   142.6 ms … 151.8 ms    20 runs

Summary
  ./sapi/cli/php toarray.php  ran
    1.29 ± 0.03 times faster than ./sapi/cli/php_old toarray.php