dom: Optimize splitText()

[ndossche]

This avoids duplicating the intermediate strings, by transferring ownership.

It's hard to measure the improvement in a reliable way, as we have to operate on the same node. The following benchmark shows a nice improvement (although not perfect as a benchmark):

<?php

$dom = new DOMDocument;
$dom->loadXML('<root>testabcdef</root>');
$text = $dom->documentElement->firstChild;

for ($i = 0; $i < 1000000; $i++) {
    $text2 = clone $text;
    $text2->splitText(5);
}

Only tested on my desktop i7-4790:

Benchmark 1: ./sapi/cli/php x.php
  Time (mean ± σ):     284.1 ms ±   2.8 ms    [User: 280.0 ms, System: 3.0 ms]
  Range (min … max):   281.4 ms … 291.3 ms    10 runs

Benchmark 2: ./sapi/cli/php_old x.php
  Time (mean ± σ):     314.0 ms ±   7.8 ms    [User: 309.2 ms, System: 2.9 ms]
  Range (min … max):   306.5 ms … 328.0 ms    10 runs

Summary
  ./sapi/cli/php x.php ran
    1.11 ± 0.03 times faster than ./sapi/cli/php_old x.php