Add a register allocation context module.

Collect the data structures that hang around between function
compilations.

Provide a main entry point to the register allocator passes.

lib/cretonne/src/regalloc/mod.rs

@@ -9,3 +9,6 @@ pub mod live_value_tracker;
 pub mod coloring;
 
 mod affinity;
+mod context;
+
+pub use self::context::Context;