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

This fixes the next part of #287.

lib/codegen/src/isa/arm32/settings.rs

@@ -0,0 +1,9 @@
+//! ARM32 Settings.
+
+use settings::{self, detail, Builder};
+use std::fmt;
+
+// Include code generated by `lib/codegen/meta/gen_settings.py`. This file contains a public
+// `Flags` struct with an impl for all of the settings defined in
+// `lib/codegen/meta/isa/arm32/settings.py`.
+include!(concat!(env!("OUT_DIR"), "/settings-arm32.rs"));