Visit downtown New Britain and you'll see a change. The city of 73,000, consigned to the New England rust belt after most of its factories closed and two highways split the city, is making a comeback. Business leaders cite the CTfastrak rapid bus system, transit-oriented developments and pedestrian-friendly streets as signs of a nascent, but visible rebirth. They also credit Avner Krohn, a 35-year-old developer from Long Island.