Cleanup and optimize attribute value reading

[ndossche]

When the attribute has a single text child, we can avoid an allocating call to libxml2 and read the contents directly.

On my i7-4790, I tested the optimization with both the $value and $nodeValue property.

Summary
  ./sapi/cli/php bench_value.php ran
    1.82 ± 0.09 times faster than ./sapi/cli/php_old bench_value.php

Summary
  ./sapi/cli/php bench_nodeValue.php ran
    1.78 ± 0.10 times faster than ./sapi/cli/php_old bench_nodeValue.php

Test code:

$dom = new DOMDocument;
$dom->loadXML('<root attrib="this is a relatively short text"/>');
$attrib = $dom->documentElement->attributes[0];

for ($i=0; $i<1000*1000; $i++) {
	$attrib->value; // or nodeValue
}