refactor(library/standard/unit): make unit type similar to the one in the hott library
refactor(library/standard/unit): make unit type similar to the one in the hott library

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

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