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/entity_list.rs

@@ -46,12 +46,11 @@
 //! The index stored in an `EntityList` points to part 2, the list elements. The value 0 is
 //! reserved for the empty list which isn't allocated in the vector.
 
+use entity_ref::EntityRef;
 use std::hash::{Hash, Hasher};
 use std::marker::PhantomData;
 use std::mem;
 
-use entity_map::EntityRef;
-
 /// A small list of entity references allocated from a pool.
 ///
 /// All of the list methods that take a pool reference must be given the same pool reference every
@@ -484,7 +483,7 @@ mod tests {
     use super::*;
     use super::{sclass_size, sclass_for_length};
     use ir::Inst;
-    use entity_map::EntityRef;
+    use entity_ref::EntityRef;
 
     #[test]
     fn size_classes() {