fix(library/num): bug in to_pos_num, missing test
fix(library/num): bug in to_pos_num, missing test

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

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