feat(util/list_fn): add get_ith function
feat(util/list_fn): add get_ith function

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

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