Allow json_decode() to return an ArrayObject

[duncan3dc]

This adds a new flag JSON_AS_ARRAY_OBJECT for json_decode(), to have it return the result as an ArrayObject instead of stdClass. This is so that it conforms to the iterable type hint.

function iter(iterable $items)
{
    foreach ($items as $key => $item) {
        echo "[{$key}] = '{$item}'" . PHP_EOL;
    }
}

$result = json_decode('{"one": "1", "two": "2"}', false, 512, JSON_AS_ARRAY_OBJECT);
iter($result);

I'll be drafting an RFC once the list is a little quieter