Richard Rodgers and Oscar Hammerstein II were riding high in 1947 on the successes of Oklahoma! and Carousel. For their next show they tried something with a Greek chorus and a stage with no sets and few props. The subject matter was unusual, too, a man whose story ends with him in a state of existential confusion. The show, Allegro, was not a hit. But now, director John Doyle has revived Allegro in a production for Classic Stage Company.