After a summer of pandering mediocrity, it's gratifying to discover a new director who is this eager to kick ass. Tim Robbins inaugurates the fall season with a slashingly funny political satire that ranks with the year's best films. "Bob Roberts" adds up to a hat-trick victory for its writer, director and star. This is clearly the year of Tim Robbins. In two richly observed performances, he has incarnated the evil twins of media and politics. As studio exec Griffin Mill in Robert Altman's