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

This fixes the next part of #287.

lib/frontend/src/variable.rs

@@ -5,7 +5,7 @@
 //! their own index types to use them directly. Frontends which don't
 //! can use the `Variable` defined here.
 
-use cretonne::entity::EntityRef;
+use cretonne_codegen::entity::EntityRef;
 use std::u32;
 
 ///! An opaque reference to a variable.