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/isa/registers.rs

@@ -1,6 +1,6 @@
 //! Data structures describing the registers in an ISA.
 
-use entity_ref::EntityRef;
+use entity::EntityRef;
 use std::fmt;
 
 /// Register units are the smallest units of register allocation.