Optimize ext/dom $wholeText

[nielsdos]

Use our own string builder instead of using libxml's and then having to copy over.

For the following test:

$dom = Dom\HTMLDocument::createEmpty();
$root = $dom->appendChild($dom->createElement('root'));
$root->append('abc', 'def', 'ghi');
$f = $root->firstChild;

for ($i = 0; $i < 1000000; $i++)
	$f->wholeText;

The following results were obtained on an i7-4790:

Benchmark 1: ./sapi/cli/php x.php
  Time (mean ± σ):      57.2 ms ±   2.3 ms    [User: 53.2 ms, System: 3.4 ms]
  Range (min … max):    54.7 ms …  69.3 ms    52 runs

Benchmark 2: ./sapi/cli/php_old x.php
  Time (mean ± σ):      89.4 ms ±   3.4 ms    [User: 85.6 ms, System: 3.0 ms]
  Range (min … max):    86.1 ms … 105.8 ms    32 runs

Summary
  ./sapi/cli/php x.php ran
    1.56 ± 0.09 times faster than ./sapi/cli/php_old x.php

[SakiTakamachi]

LGTM

[devnexen]

nice !