Rename the entity_ref module to 'entity'.

This is in preparation for moving a number of data structures into
sub-modules of the 'entity' module:

- EntityList
- EntityMap
- SparseMap

lib/cretonne/src/lib.rs

@@ -13,7 +13,7 @@ pub const VERSION: &'static str = env!("CARGO_PKG_VERSION");
 #[macro_use]
 pub mod dbg;
 #[macro_use]
-pub mod entity_ref;
+pub mod entity;
 
 pub mod binemit;
 pub mod bitset;