Add an entity_map module.

This supports the pattern of creating structs wrapping a u32 and using them as
indexes into a vector of entities. These entity references should implement the
EntityRef trait.

The EntityMap is a generic map from an EntityRef to some value type. It expects
densely indexed entities and uses a Vec to represent the mapping compactly.

src/libcretonne/lib.rs

@@ -15,3 +15,5 @@ pub mod instructions;
 pub mod repr;
 pub mod write;
 pub mod cfg;
+
+pub mod entity_map;