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

This fixes the next part of #287.

lib/codegen/src/ref_slice.rs

@@ -0,0 +1,18 @@
+//! Functions for converting a reference into a singleton slice.
+//!
+//! See also the [`ref_slice` crate](https://crates.io/crates/ref_slice).
+//!
+//! We define the functions here to avoid external dependencies, and to ensure that they are
+//! inlined in this crate.
+//!
+//! Despite their using an unsafe block, these functions are completely safe.
+
+use std::slice;
+
+pub fn ref_slice<T>(s: &T) -> &[T] {
+    unsafe { slice::from_raw_parts(s, 1) }
+}
+
+pub fn ref_slice_mut<T>(s: &mut T) -> &mut [T] {
+    unsafe { slice::from_raw_parts_mut(s, 1) }
+}