Move EntityRef and entity_impl! into a new module.

The EntityRef trait is used by more than just the EntityMap now, so it
should live in its own module.

Also move the entity_impl! macro into the new module so it can be used
for defining new entity references anywhere.

lib/cretonne/src/ir/jumptable.rs

@@ -122,7 +122,7 @@ impl Display for JumpTableData {
 mod tests {
     use super::JumpTableData;
     use ir::Ebb;
-    use entity_map::EntityRef;
+    use entity_ref::EntityRef;
 
     #[test]
     fn empty() {