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/ir/progpoint.rs

@@ -1,6 +1,6 @@
 //! Program points.
 
-use entity_map::EntityRef;
+use entity_ref::EntityRef;
 use ir::{Ebb, Inst, ValueDef};
 use std::fmt;
 use std::u32;
@@ -122,7 +122,7 @@ pub trait ProgramOrder {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use entity_map::EntityRef;
+    use entity_ref::EntityRef;
     use ir::{Inst, Ebb};
 
     #[test]