Reenable usage of nqp::nativecast, and its tests

This reverts commit c0f68bca0a3d34a7d59f872fce0e7f34892cdff2.
bind result of nqp::nativecallcast so there is no extra container

Otherwise calling nqp::isint($result) will blow up.
box native string returned by nativecallcast when we get one
simplify sub nativecast

We do not need to box the return value anymore since we provide the proper
box target now. A call looks now like: nativecast(int32, Int, $opaque-pointer)