Add entity lists.

Like a vector, but with a tiny footprint, and allocated from a pool so
all memory can be released very quickly.

lib/cretonne/src/lib.rs

@@ -14,6 +14,7 @@ pub mod isa;
 pub mod cfg;
 pub mod dominator_tree;
 pub mod entity_map;
+pub mod entity_list;
 pub mod sparse_map;
 pub mod settings;
 pub mod verifier;