Add a LiveRange data structure.

We will track live ranges separately for each SSA value, rather than per
virtual register like LLVM does.

This is the basis for a register allocator, so place it in a new
regalloc module.

lib/cretonne/src/lib.rs

@@ -16,6 +16,7 @@ pub mod dominator_tree;
 pub mod entity_map;
 pub mod settings;
 pub mod verifier;
+pub mod regalloc;
 
 mod write;
 mod constant_hash;