feat(library/data/nat/pairing): add "elegant" pairing/unpairing function
feat(library/data/nat/pairing): add "elegant" pairing/unpairing function

johoelzl/lean,soonhokong/lean-osx,fpvandoorn/lean,javra/lean,digama0/lean,javra/lean,sp3ctum/lean,digama0/lean,Kha/lean,fpvandoorn/lean,avigad/lean,levnach/lean,johoelzl/lean,johoelzl/lean,rlewis1988/lean,fgdorais/lean,eigengrau/lean,rlewis1988/lean,htzh/lean,Kha/lean,eigengrau/lean,javra/lean,htzh/lean,rlewis1988/lean,dselsam/lean,leodemoura/lean,Kha/lean,levnach/lean,htzh/lean,fpvandoorn/lean,sp3ctum/lean,soonhokong/lean-osx,fpvandoorn/lean2,leanprover-community/lean,UlrikBuchholtz/lean,fpvandoorn/lean,eigengrau/lean,levnach/lean,c-cube/lean,digama0/lean,digama0/lean,levnach/lean,leanprover/lean,sp3ctum/lean,fpvandoorn/lean2,johoelzl/lean,leanprover-community/lean,leanprover-community/lean,rlewis1988/lean,dselsam/lean,UlrikBuchholtz/lean,eigengrau/lean,rlewis1988/lean,soonhokong/lean,javra/lean,c-cube/lean,avigad/lean,fgdorais/lean,soonhokong/lean-windows,c-cube/lean,fgdorais/lean,dselsam/lean,leanprover-community/lean,soonhokong/lean,UlrikBuchholtz/lean,soonhokong/lean,dselsam/lean,UlrikBuchholtz/lean,levnach/lean,dselsam/lean,soonhokong/lean-osx,leodemoura/lean,leodemoura/lean,leanprover/lean,leanprover-community/lean,leodemoura/lean,johoelzl/lean,digama0/lean,fgdorais/lean,avigad/lean,eigengrau/lean,sp3ctum/lean,fgdorais/lean,johoelzl/lean,avigad/lean,soonhokong/lean-osx,leanprover/lean,rlewis1988/lean,Kha/lean,fpvandoorn/lean2,htzh/lean,soonhokong/lean-windows,leodemoura/lean,dselsam/lean,leanprover-community/lean,javra/lean,leanprover/lean,soonhokong/lean-windows,avigad/lean,fgdorais/lean,c-cube/lean,fpvandoorn/lean,soonhokong/lean,soonhokong/lean-windows,digama0/lean,UlrikBuchholtz/lean,fpvandoorn/lean2,Kha/lean,leodemoura/lean,c-cube/lean,fpvandoorn/lean2,leanprover/lean,fpvandoorn/lean,soonhokong/lean-osx,soonhokong/lean-windows,avigad/lean,htzh/lean,sp3ctum/lean,soonhokong/lean,leanprover/lean,Kha/lean