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

This fixes the next part of #287.

lib/filetests/src/test_print_cfg.rs

@@ -5,8 +5,8 @@
 
 use std::borrow::Cow;
 
-use cretonne::cfg_printer::CFGPrinter;
-use cretonne::ir::Function;
+use cretonne_codegen::cfg_printer::CFGPrinter;
+use cretonne_codegen::ir::Function;
 use cretonne_reader::TestCommand;
 use subtest::{self, Context, Result as STResult, SubTest};