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

This fixes the next part of #287.

lib/codegen/src/isa/arm64/enc_tables.rs

@@ -0,0 +1,10 @@
+//! Encoding tables for ARM64 ISA.
+
+use ir;
+use isa;
+use isa::constraints::*;
+use isa::enc_tables::*;
+use isa::encoding::RecipeSizing;
+
+include!(concat!(env!("OUT_DIR"), "/encoding-arm64.rs"));
+include!(concat!(env!("OUT_DIR"), "/legalize-arm64.rs"));