Implement nqp::atpos, nqp::push, nqp::pop, nqp::shift.
Implement nqp::atpos, nqp::push, nqp::pop, nqp::shift.

Add add_sideffect_op helper so that we can define ops with sideffects easily.

cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp