导出和导入宏

导出宏以允许其他模块使用它：

    #[macro_export]
//  ^~~~~~~~~~~~~~~ Think of it as `pub` for macros.
    macro_rules! my_macro { (..) => {..} }

使用来自其他包装箱或模块的宏：

   #[macro_use] extern crate lazy_static;
// ^~~~~~~~~~~~ Must add this in order to use macros from other crates 

   #[macro_use] mod macros;
// ^~~~~~~~~~~~ The same for child modules.