To most Americans, ALISTAIR COOKE was the baronial M.C. of Masterpiece Theatre a genial gent so famous that he was gently parodied as Alistair Cookie on Sesame Street and Alistair Beagle in Peanuts. But his role as a TV host was a sideline for the British journalist who knew everyone and remembered everything. As a young Cambridge grad in the U.S., he became instant pals with Charlie Chaplin and H.L. Mencken. He got his first glimpse of Franklin Roosevelt as the paraplegic President was hauled from his car, and he happened to be near Robert Kennedy the night of his assassination. All these encounters, and thousands more, he related in a weekly BBC chat series, Letter from America, that mesmerized millions of listeners on five continents and ran for 58 years the world's longest-running series with a single person as host. When Cooke died last week at 95, the English-speaking world said a melancholy good-night to a consummate reporter and the world's wisest, blithest uncle.