Improve support of mixed type in objects

[Jeckerson]

public function offsetGet(mixed index) -> mixed
{
    return this->{index};
}

generates error:

[ERROR] Variable type: mixed cannot be used in property-dynamic-access in /srv/phalcon/Mvc/Model/Row.zep on line 43

Also

public function offsetExists(mixed index) -> bool
{
    return isset this->{index};
}

generates error:

[ERROR] [mixed] in /srv/phalcon/Mvc/Model/Row.zep on line 55

[Jeckerson]

Cast from mixed to string also doesn't work:

[ERROR] Unknown type: mixed in /srv/phalcon/Support/Collection.zep on line 232