feat(library/num): handle nat.zero and (nat.succ nat.zero) at to_num
feat(library/num): handle nat.zero and (nat.succ nat.zero) at to_num

rlewis1988/lean,johoelzl/lean,fpvandoorn/lean,leodemoura/lean,johoelzl/lean,rlewis1988/lean,leanprover-community/lean,avigad/lean,leanprover/lean,avigad/lean,fpvandoorn/lean,leanprover/lean,Kha/lean,johoelzl/lean,fgdorais/lean,leanprover/lean,fpvandoorn/lean,rlewis1988/lean,leanprover-community/lean,leodemoura/lean,dselsam/lean,fgdorais/lean,digama0/lean,fpvandoorn/lean,leodemoura/lean,johoelzl/lean,dselsam/lean,avigad/lean,avigad/lean,leodemoura/lean,avigad/lean,dselsam/lean,digama0/lean,johoelzl/lean,digama0/lean,leanprover-community/lean,Kha/lean,Kha/lean,leanprover-community/lean,leanprover/lean,fgdorais/lean,rlewis1988/lean,fpvandoorn/lean,dselsam/lean,leanprover/lean,avigad/lean,Kha/lean,leanprover/lean,Kha/lean,dselsam/lean,leodemoura/lean,leodemoura/lean,digama0/lean,johoelzl/lean,digama0/lean,fgdorais/lean,rlewis1988/lean,leanprover-community/lean,digama0/lean,fgdorais/lean,rlewis1988/lean,dselsam/lean,Kha/lean,leanprover-community/lean,fpvandoorn/lean,fgdorais/lean