Rename the 'cretonne' crate to 'cretonne-codegen'.

This fixes the next part of #287.

lib/codegen/src/regalloc/mod.rs

@@ -0,0 +1,23 @@
+//! Register allocation.
+//!
+//! This module contains data structures and algorithms used for register allocation.
+
+pub mod register_set;
+pub mod coloring;
+pub mod live_value_tracker;
+pub mod liveness;
+pub mod liverange;
+pub mod virtregs;
+
+mod affinity;
+mod coalescing;
+mod context;
+mod diversion;
+mod pressure;
+mod reload;
+mod solver;
+mod spilling;
+
+pub use self::register_set::RegisterSet;
+pub use self::context::Context;
+pub use self::diversion::RegDiversions;