u3: factors out "advance" block of u3a_take()
u3: factors out "advance" block of u3a_take()

ngzax/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,urbit/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,urbit/urbit