feat(library/data/list/set.lean): add two theorems
feat(library/data/list/set.lean): add two theorems

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