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

This fixes the next part of #287.

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

@@ -0,0 +1,15 @@
+"""
+ARM64 definitions.
+
+Commonly used definitions.
+"""
+from __future__ import absolute_import
+from cdsl.isa import TargetISA, CPUMode
+import base.instructions
+from base.legalize import narrow
+
+ISA = TargetISA('arm64', [base.instructions.GROUP])
+A64 = CPUMode('A64', ISA)
+
+# TODO: Refine these
+A64.legalize_type(narrow)