HAVANA, Cuba — Latin America has long been a case study in the social ills brought by sharp economic inequality, its class-stratified societies marked by too few haves and too many have-nots. But even as income divisions widen across the United States and much of the developing world, they are narrowing in Latin America. Poverty in the region is at its lowest point in decades, according to several new reports, and millions are moving upward into the middle class.