Unknown and unheralded in the U.S., French Pantomimist Marcel Marceau, 32, opened last week in Manhattan (off Broadway) for a two-week run. When the curtain rose on a bare stage and a black backdrop, it looked as if Mime Marceau, gesticulating but wordless, had about as much chance of success in hard-to-please New York as a mute at a hog-calling contest. But next morning the critics called him "superb," his work a "masterpiece."