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/lib.rs

@@ -12,6 +12,8 @@ pub const VERSION: &'static str = env!("CARGO_PKG_VERSION");
 
 #[macro_use]
 pub mod dbg;
+#[macro_use]
+pub mod entity_ref;
 
 pub mod binemit;
 pub mod dominator_tree;