Saab is on a roll. Sales have been booming in recent years, as a popular chord with all the models seems to have been struck. The turbocharged 9-5 Aero, introduced last fall, is an expression of Saab's confidence. It's the fastest and hottest Saab ever built, and it's good. With the Aero, Saab is making a credible statement that it can play with the big boys-and it is also selling cars.