perf(kernel/for_each): delay initialization of visited set
perf(kernel/for_each): delay initialization of visited set

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

fpvandoorn/lean2,levnach/lean,codyroux/lean0.1,soonhokong/travis_test,fpvandoorn/lean,fgdorais/lean,levnach/lean,johoelzl/lean,leodemoura/lean,dselsam/lean,soonhokong/lean-windows,soonhokong/lean,leanprover/lean,eigengrau/lean,UlrikBuchholtz/lean,soonhokong/lean,soonhokong/lean-osx,c-cube/lean,soonhokong/lean-windows,htzh/lean,soonhokong/lean-osx,johoelzl/lean,fgdorais/lean,leanprover/lean,leanprover-community/lean,fpvandoorn/lean2,codyroux/lean0.1,leodemoura/lean,javra/lean,leanprover-community/lean,htzh/lean,leanprover-community/lean,avigad/lean,c-cube/lean,soonhokong/lean-windows,fpvandoorn/lean,fpvandoorn/lean,Kha/lean,fpvandoorn/lean2,digama0/lean,fpvandoorn/lean,dselsam/lean,soonhokong/travis_test,c-cube/lean,javra/lean,eigengrau/lean,fgdorais/lean,eigengrau/lean,eigengrau/lean,c-cube/lean,digama0/lean,sp3ctum/lean,rlewis1988/lean,dselsam/lean,soonhokong/lean-osx,leodemoura/lean,leanprover/lean,leanprover/lean,levnach/lean,digama0/lean,sp3ctum/lean,soonhokong/lean-osx,soonhokong/lean,codyroux/lean0.1,soonhokong/lean-windows,fgdorais/lean,eigengrau/lean,sp3ctum/lean,leanprover-community/lean,johoelzl/lean,soonhokong/travis_test,johoelzl/lean,johoelzl/lean,fgdorais/lean,htzh/lean,Kha/lean,UlrikBuchholtz/lean,fpvandoorn/lean,digama0/lean,rlewis1988/lean,rlewis1988/lean,soonhokong/lean,c-cube/lean,htzh/lean,soonhokong/travis_test,leodemoura/lean,sp3ctum/lean,soonhokong/lean,rlewis1988/lean,rlewis1988/lean,javra/lean,avigad/lean,soonhokong/lean-osx,johoelzl/lean,leanprover-community/lean,leanprover/lean,levnach/lean,avigad/lean,Kha/lean,UlrikBuchholtz/lean,leanprover/lean,htzh/lean,sp3ctum/lean,javra/lean,leodemoura/lean,Kha/lean,UlrikBuchholtz/lean,avigad/lean,leodemoura/lean,fpvandoorn/lean,fgdorais/lean,soonhokong/lean-windows,Kha/lean,dselsam/lean,codyroux/lean0.1,fpvandoorn/lean2,avigad/lean,digama0/lean,rlewis1988/lean,UlrikBuchholtz/lean,leanprover-community/lean,Kha/lean,dselsam/lean,dselsam/lean,levnach/lean,avigad/lean,fpvandoorn/lean2,digama0/lean,javra/lean