chore(library/standard): port (an older version of) Floris's nat library to Lean 0.2
chore(library/standard): port (an older version of) Floris's nat library to Lean 0.2

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

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