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/layout.rs

@@ -982,7 +982,7 @@ impl<'f> Cursor<'f> {
 #[cfg(test)]
 mod tests {
     use super::{Layout, Cursor, CursorPosition};
-    use entity_map::EntityRef;
+    use entity_ref::EntityRef;
     use ir::{Ebb, Inst, ProgramOrder};
     use std::cmp::Ordering;