moved of_g to c_gen.of_g_legacy, introduced loxi_globals

as of_g includes loads of legacy data (and a representation that
is increasingly tailored to the c backend only), I've moved it to
c_gen.of_g. In its place, I have created loxi_globals, which gives
access to the list of versions and the IR.

Also loxi reorg: move command line processing to cmdline.py
diff --git a/c_gen/templates/locitest/test_validator.c b/c_gen/templates/locitest/test_validator.c
index eb3cbb6..a4b55eb 100644
--- a/c_gen/templates/locitest/test_validator.c
+++ b/c_gen/templates/locitest/test_validator.c
@@ -26,7 +26,7 @@
 :: # under the EPL.
 ::
 :: include('_copyright.c')
-:: import of_g
+:: import c_gen.of_g_legacy as of_g
 :: import c_gen.loxi_utils_legacy as loxi_utils
 :: from c_gen import type_maps