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/isa/registers.rs

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