feat(library/standard): add hilbert's choice operator
feat(library/standard): add hilbert's choice operator

Signed-off-by: Leonardo de Moura <7610bae85f2b530654cc716772f1fe653373e892@microsoft.com>

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