Panama's first woman president was due to take power last Wednesday, four months before the tiny nation takes control of one of the world's most important waterways, the Panama Canal. Mireya Moscoso, the 53-year-old widow of three-time Panamanian President Arnulfo Arias, won the May elections with 45 percent of the vote, throwing her predecessor, Ernesto Perez Balladares, out of power and pledging to fight poverty and unemployment in the country of 2.7 million people. Moscoso, a teacher's daughter from the agricultural heartland, has never held elective office. But since her husband's death in 1988, she assumed increasing prominence in the Arnulfista party named for her late husband.