fix: implement "Copy" to use enum in union

Author: manhatsu

src/coding-guidelines/types-and-traits.rst

@@ -114,6 +114,7 @@ Types and Traits
       .. code-block:: rust
 
          #[repr(u8)]
+         #[derive(Copy, Clone)]
          enum Color {
              Red = 0,
              Green = 1,