匯出和匯入巨集

匯出巨集以允許其他模組使用它：

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

使用來自其他包裝箱或模組的巨集：

   #[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.