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/liverange.rs

@@ -435,7 +435,7 @@ impl SparseMapValue<Value> for LiveRange {
 mod tests {
     use super::LiveRange;
     use ir::{Inst, Ebb, Value};
-    use entity_ref::EntityRef;
+    use entity::EntityRef;
     use ir::{ProgramOrder, ExpandedProgramPoint};
     use std::cmp::Ordering;