Optimize descendant-or-self and child axes.
Optimize descendant-or-self and child axes.

By using NodeSet#concat we can further reduce the amount of object allocations.
This in turn greatly reduces the time it takes to query large documents using
descendant-or-self.

jeffreybaird/oga,dfockler/oga,YorickPeterse/oga,jeffreybaird/oga,YorickPeterse/oga,altmetric/oga,jeffreybaird/oga,dfockler/oga,YorickPeterse/oga,ttasanen/oga,jeffreybaird/oga,dfockler/oga,dfockler/oga,YorickPeterse/oga,ttasanen/oga,jeffreybaird/oga,ttasanen/oga,altmetric/oga,ttasanen/oga,altmetric/oga,altmetric/oga,dfockler/oga,altmetric/oga,ttasanen/oga,YorickPeterse/oga