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

This fixes the next part of #287.

cranelift/src/utils.rs

@@ -1,8 +1,8 @@
 //! Utility functions.
 
-use cretonne::isa;
-use cretonne::isa::TargetIsa;
-use cretonne::settings::{self, FlagsOrIsa};
+use cretonne_codegen::isa;
+use cretonne_codegen::isa::TargetIsa;
+use cretonne_codegen::settings::{self, FlagsOrIsa};
 use cretonne_reader::{parse_options, Location};
 use std::fs::File;
 use std::io::{self, Read};