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

This fixes the next part of #287.

lib/codegen/meta/test_srcgen.py

@@ -0,0 +1,8 @@
+from __future__ import absolute_import
+import doctest
+import srcgen
+
+
+def load_tests(loader, tests, ignore):
+    tests.addTests(doctest.DocTestSuite(srcgen))
+    return tests