SSA register coloring pass.

This is a bare-bones outline of the SSA coloring pass. Many features are
missing, including:

- Handling instruction operand constraints beyond simple register
  classes.
- Handling ABI requirements for function arguments and return values.
- Generating shuffle code for EBB arguments.

lib/cretonne/src/regalloc/mod.rs

@@ -6,5 +6,6 @@ pub mod liverange;
 pub mod liveness;
 pub mod allocatable_set;
 pub mod live_value_tracker;
+pub mod coloring;
 
 mod affinity;