Derive Debug for Flag<B>

This just makes it possible to easily print the `FLAGS` constant.

src/traits.rs

@@ -11,6 +11,7 @@ use crate::{
 /**
 A defined flags value that may be named or unnamed.
 */
+#[derive(Debug)]
 pub struct Flag<B> {
     name: &'static str,
     value: B,