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

This fixes the next part of #287.

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

@@ -0,0 +1,7 @@
+//! Emitting binary ARM32 machine code.
+
+use binemit::{bad_encoding, CodeSink};
+use ir::{Function, Inst};
+use regalloc::RegDiversions;
+
+include!(concat!(env!("OUT_DIR"), "/binemit-arm32.rs"));