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/regalloc/virtregs.rs

@@ -18,7 +18,7 @@ use packed_option::PackedOption;
 use ref_slice::ref_slice;
 
 /// A virtual register reference.
-#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, PartialOrd, Ord)]
+#[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
 pub struct VirtReg(u32);
 entity_impl!(VirtReg, "vreg");