Trade and investment between China and the Association of Southeast Asian Nations are expected to more than double by 2020, Premier Li Keqiang said on Tuesday.   These goals are part of what the premier calls a "diamond decade" of relations the two must strive to achieve.   Trade is expected to reach $1 trillion in 2020, compared with $400 billion last year, Li said at the 10th China-ASEAN Expo, which opened on Tuesday in Nanning, capital of the Guangxi Zhuang autonomous re...