ISSUE：支持monad语法

对于拥有下面两个方法的类型T<E, [anything...]>，可构成monad:

func bind<T, E>(self: T<E,[anything...]>, f:(E)=> T<E, [anything...]>)=> T<E, [anything...]>;
func box<E>(elem: E)=> T<E, [anything...]>;

编译器将提供monad相关的运算符以及语法，目前尚无方案