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

This fixes the next part of #287.

lib/filetests/src/test_verifier.rs

@@ -9,8 +9,8 @@
 //! This annotation means that the verifier is expected to given an error for the jump instruction
 //! containing the substring "jump to non-existent EBB".
 
-use cretonne::ir::Function;
-use cretonne::verify_function;
+use cretonne_codegen::ir::Function;
+use cretonne_codegen::verify_function;
 use cretonne_reader::TestCommand;
 use match_directive::match_directive;
 use std::borrow::{Borrow, Cow};