Watching "The Jerk," a comedy starring Steve Martin, is like spending an afternoon on the rickety rides of some sleazo travelling carnival …: now and then, an involuntary laugh is bumped out of us by an unexpected lurch or spin of the Whip, but most of the attractions are so slow and noisy and uncomfortable that even while we continue to smile (we're here for fun, aren't we?) we are mostly aware of the encompassing smells and dirt, and of the dimness of effort that has gone into the whole feeble entertainment. Steve Martin, of course, has been a star turn on television's "Saturday Night Live," but "The Jerk," of which he is co-author, owes much more to Mel Brooks. I am not a perfectly uncritical Brooks fan, but compared to this movie his "Blazing Saddles" and "High Anxiety" are super-rides, from which we emerge with a gasping "Wow...