THERE are a number of small, unexpectedly funny moments in "A Christmas Story," but you have to possess the stamina of a pearl diver to find them. The film, which opens today at the Guild and other theaters, is about growing up in the early 1940's in a middle-size city in Indiana. In particular it's about 9-year old Ralphie Parker (Peter Billingsley) who, more than anything else in the world, wants to own a genuine Red Ryder air rifle, one that comes complete with a shockproof compass and a sundial set into its stock...