feat(util/lua): add macro pushnil
feat(util/lua): add macro pushnil

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

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