Implement a SparseMap data structure.

This implements the classic Briggs/Torczon sparse set construct.

Adapt it to our existing EntityRef infrastructure so we can use types
keys instead of just integers like the original paper does.

Also provide a SparseSet<T> type alias which implements a sparse set of
entity refeences. This is actually closer to what the original paper
describes.

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 sparse_map;
 pub mod settings;
 pub mod verifier;
 pub mod regalloc;