Power function as an operator.

This PR complements the [pow-operator](https://wiki.php.net/rfc/pow-operator) proposal, which will soon reach its voting phase.

The power operator allows expressions, like:

    echo 2 ** 3; // 8
    $x = 2;
    echo $x **= 3; // 8

It also supports GMP overloading:

    $x = gmp_init(2) ** 3;
    var_dump($x instanceof GMP); // bool(true)