Optimize DOM property access

[ndossche]

For the read and write implementation, store the handler pointer in the first cache slot.
For the write implementation, use the second cache slot to store the property info.

For a micro-benchmark that performs a write:

$dom = new DOMDocument;
for ($i=0;$i<9999999;$i++)
        $dom->strictErrorChecking = false;

I obtain the following results on an i7-4790:

  ./sapi/cli/php ./write.php ran
    1.42 ± 0.08 times faster than ./sapi/cli/php_old ./write.php

For a micro-benchmark that performs a read:

$dom = new DOMDocument;
for ($i=0;$i<9999999;$i++)
        $dom->strictErrorChecking;

I obtain the following results on the same machine:

  ./sapi/cli/php ./read.php ran
    1.29 ± 0.13 times faster than ./sapi/cli/php_old ./read.php

[devnexen]

MSTM