Allow unpacking multiple arguments

PHP (5.6+) allows unpacking multiple arguments, HHVM only allows unpacking for the last argument. For example, the following does not work:

``` php
function add($a, $b, $c, $d)
{
echo $a + $b + $c + $d;
}

add(...[2, 3], ...[4, 5]);
```

http://3v4l.org/lLuMG