ONE moves markets, the other moves audience ratings. The wedding this Sunday of Federal Reserve chairman Alan Greenspan and journalist Andrea Mitchell is a merger made in Washington; the second most powerful man in the United States weds big time television personality. They met when she covered a commission created to rescue the Social Security system. He chaired the commission. Friends are taking bets on whether Greenspan "a manufacturer of verbal murk," says an old pal - will be able to say anything as direct as "I do".