After turning out two very successful plays that focus on narrowly circumscribed situations. William Gibson has given his talent an off-Broadway holiday in a cosmic fantasy titled "Dinny and the Witches." Dinny is a sort of cool Peer Gynt who along the way enchants us with one simple romantic song that admits "Don't Know What I'm Here For." The quality of this song written by Mr. Gibson, who is both an ex-piano player and contributor to poetry magazines, suggests that this new play might be much better as a musical comedy. Instead the playwright has taken much too long for his hero to find a much too simple answer to his question. The byplay consists of three comic witches, who, like the Button-Moulder, must execute Dinny's predestined death sentence. One of the three is a friendly "nitwitch" who is always in favor of "taking the next breath" and...