“Sondheim on Sondheim,” the revue put together by James Lapine from Stephen Sondheim’s songs, confirms enchantingly what we already know but can gladly bear such eloquent repeating of: that Sondheim is the best composer- lyricist we’ve got. To have it incarnated on Broadway by a cast headed by Barbara Cook, Vanessa Williams, Tom Wopat and five personable others leaves us, after two-and-a-half hours, only hungry for more. Every item in this expansive, diverse catalog seems to claim swift recognition: “I am a Sondheim song, and could be no other’s.”