I suppose, in introducing Leon Prochnik's The Existentialist, I could pound out a few hundred words on form, draw a parallel or two with earlier works, toss in the names Bunuel and Keaton, render some empty judgment on the film's place in the Cinema of its day; I could do all of those things. I guess. But . . . really, all you need to know about today's offering is this: 　　No fil...