By ANNE D'INNOCENZIO, AP Retail Writer NEW YORK (AP) — For years, teens flocked to Forever 21's massive stores at the nation's malls for its speedy take on fashion, like its $5 shimmery halter tops and $25 dresses. But the chain that helped popularize so-called fast fashion has moved too slow for a new generation of young customers. The Los Angeles-based privately held chain filed for Chapter 11 bankruptcy