Move EntityList and SparseMap into the entity module.

These data structures are dependent on EntityRef and EntityMap, so it
makes sense to keep them in the same module.

lib/cretonne/src/entity/mod.rs

@@ -6,12 +6,16 @@
 //! Various data structures based on the entity references are defined in sub-modules.
 
 mod keys;
+mod list;
 mod map;
 mod primary;
+mod sparse;
 
 pub use self::keys::Keys;
+pub use self::list::{EntityList, ListPool};
 pub use self::map::EntityMap;
 pub use self::primary::PrimaryMap;
+pub use self::sparse::{SparseSet, SparseMap, SparseMapValue};
 
 /// A type wrapping a small integer index should implement `EntityRef` so it can be used as the key
 /// of an `EntityMap` or `SparseMap`.