Many fine productions, large-scale or intimate, have sauntered their way up and down Fleet Street since Hugh Wheeler’s book and Sondheim’s score premiered in 1979 in Harold Prince’s acclaimed staging. But the current revival at the Lunt-Fontanne is a thing apart. Thomas Kail, director of that latter-day groundbreaker Hamilton, has assembled all the pieces needed for what Sondheim himself finally summed up as “a movie for the stage.” The result is something both sumptuous and terrifying, truly a Sweeney Todd for the ages.