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

This fixes the next part of #287.

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

@@ -0,0 +1,14 @@
+"""
+ARM 32-bit Architecture
+-----------------------
+
+This target ISA generates code for ARMv7 and ARMv8 CPUs in 32-bit mode
+(AArch32). We support both ARM and Thumb2 instruction encodings.
+"""
+
+from __future__ import absolute_import
+from . import defs
+from . import settings, registers  # noqa
+
+# Re-export the primary target ISA definition.
+ISA = defs.ISA.finish()