Rename the entity_ref module to 'entity'.

This is in preparation for moving a number of data structures into
sub-modules of the 'entity' module:

- EntityList
- EntityMap
- SparseMap

lib/cretonne/src/regalloc/diversion.rs

@@ -104,7 +104,7 @@ impl RegDiversions {
 mod tests {
     use super::*;
     use ir::Value;
-    use entity_ref::EntityRef;
+    use entity::EntityRef;
 
     #[test]
     fn inserts() {