Implement difference method for OrderedSet
Implement difference method for OrderedSet

This implementation is obvious rather than efficient, but it's efficient
enough for our uses I think. It uses `type(self)` to guarantee that it
works even in subclasses or if the name of the class changes.

thiblahute/meson,jpakkane/meson,mesonbuild/meson,pexip/meson,QuLogic/meson,jpakkane/meson,fmuellner/meson,fmuellner/meson,rhd/meson,pexip/meson,QuLogic/meson,jeandet/meson,QuLogic/meson,mesonbuild/meson,pexip/meson,rhd/meson,trhd/meson,MathieuDuponchelle/meson,mesonbuild/meson,wberrier/meson,wberrier/meson,ernestask/meson,thiblahute/meson,trhd/meson,wberrier/meson,becm/meson,aaronp24/meson,aaronp24/meson,ernestask/meson,mesonbuild/meson,MathieuDuponchelle/meson,aaronp24/meson,QuLogic/meson,ernestask/meson,QuLogic/meson,ernestask/meson,mesonbuild/meson,jeandet/meson,jpakkane/meson,pexip/meson,becm/meson,becm/meson,MathieuDuponchelle/meson,thiblahute/meson,becm/meson,ernestask/meson,MathieuDuponchelle/meson,fmuellner/meson,jpakkane/meson,rhd/meson,ernestask/meson,thiblahute/meson,trhd/meson,MathieuDuponchelle/meson,becm/meson,fmuellner/meson,becm/meson,trhd/meson,jpakkane/meson,jeandet/meson,becm/meson,jpakkane/meson,wberrier/meson,jeandet/meson,ernestask/meson,trhd/meson,mesonbuild/meson,jeandet/meson,wberrier/meson,jeandet/meson,trhd/meson,fmuellner/meson,aaronp24/meson,aaronp24/meson,rhd/meson,jpakkane/meson,becm/meson,mesonbuild/meson,trhd/meson,aaronp24/meson,fmuellner/meson,trhd/meson,QuLogic/meson,MathieuDuponchelle/meson,pexip/meson,thiblahute/meson,trhd/meson,pexip/meson,mesonbuild/meson,rhd/meson,pexip/meson,QuLogic/meson,ernestask/meson,rhd/meson,thiblahute/meson,jeandet/meson,jpakkane/meson,QuLogic/meson,MathieuDuponchelle/meson,fmuellner/meson,wberrier/meson,jeandet/meson,ernestask/meson,aaronp24/meson,fmuellner/meson,becm/meson,MathieuDuponchelle/meson,wberrier/meson,rhd/meson,QuLogic/meson,thiblahute/meson,fmuellner/meson,aaronp24/meson,thiblahute/meson,rhd/meson,pexip/meson,jpakkane/meson,jeandet/meson,thiblahute/meson,MathieuDuponchelle/meson,becm/meson,wberrier/meson,pexip/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,aaronp24/meson