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/ir/progpoint.rs

@@ -1,6 +1,6 @@
 //! Program points.
 
-use entity_ref::EntityRef;
+use entity::EntityRef;
 use ir::{Ebb, Inst, ValueDef};
 use std::fmt;
 use std::u32;
@@ -135,7 +135,7 @@ pub trait ProgramOrder {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use entity_ref::EntityRef;
+    use entity::EntityRef;
     use ir::{Inst, Ebb};
 
     #[test]