feat(library/data/finset/comb): define powerset
feat(library/data/finset/comb): define powerset

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