Provide an fmt::Debug impl for entity references.

Instead of deriving a Debug impl: Ebb(45), use the Display version for
Debug too: ebb45.

This is more readable, and no information is lost.

lib/cretonne/src/bforest/mod.rs

@@ -97,7 +97,7 @@ trait Forest {
 }
 
 /// A reference to a B+-tree node.
-#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+#[derive(Clone, Copy, PartialEq, Eq)]
 struct Node(u32);
 entity_impl!(Node, "node");