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

This fixes the next part of #287.

lib/codegen/meta/isa/arm32/defs.py

@@ -0,0 +1,19 @@
+"""
+ARM 32-bit 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('arm32', [base.instructions.GROUP])
+
+# CPU modes for 32-bit ARM and Thumb2.
+A32 = CPUMode('A32', ISA)
+T32 = CPUMode('T32', ISA)
+
+# TODO: Refine these.
+A32.legalize_type(narrow)
+T32.legalize_type(narrow)