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

This fixes the next part of #287.

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

@@ -0,0 +1,11 @@
+"""
+ARM32 settings.
+"""
+from __future__ import absolute_import
+from cdsl.settings import SettingGroup
+import base.settings as shared
+from .defs import ISA
+
+ISA.settings = SettingGroup('arm32', parent=shared.group)
+
+ISA.settings.close(globals())