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

This fixes the next part of #287.

lib/codegen/meta/isa/arm64/__init__.py

@@ -0,0 +1,13 @@
+"""
+ARM 64-bit Architecture
+-----------------------
+
+ARMv8 CPUs running the Aarch64 architecture.
+"""
+
+from __future__ import absolute_import
+from . import defs
+from . import settings, registers  # noqa
+
+# Re-export the primary target ISA definition.
+ISA = defs.ISA.finish()