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

This fixes the next part of #287.

lib/filetests/src/subtest.rs

@@ -1,8 +1,8 @@
 //! `SubTest` trait.
 
-use cretonne::ir::Function;
-use cretonne::isa::TargetIsa;
-use cretonne::settings::{Flags, FlagsOrIsa};
+use cretonne_codegen::ir::Function;
+use cretonne_codegen::isa::TargetIsa;
+use cretonne_codegen::settings::{Flags, FlagsOrIsa};
 use cretonne_reader::{Comment, Details};
 use filecheck::{Checker, CheckerBuilder, NO_VARIABLES};
 use std::borrow::Cow;