Seven Dials Playhouse, LondonThe playwright's relationship with Kenneth Halliwell is given new clarity in a play that is both hilarious and chilling 'I've high hopes of dying young," announces Joe Orton cheerfully in Diary of a Somebody. He got his wish: the author of barbed, subversive comedies such as Entertaining Mr Sloane and Loot was murdered in 1967 at the age of 34 by his lover Kenneth Halliwell. This play, pi…