LONDON – A great deal of water has flowed through the Taiwan Strait in the 70 years since the leader of China’s Communists, Mao Zedong, met the leader of his nationalist opponents, Chiang Kai-shek. So the recent meeting in Singapore between their heirs, President Xi Jinping of the People’s Republic and his Taiwanese counterpart, Ma Ying-jeou, could legitimately be described as historic.