Implement support for multi-module export

This add support for exporting constants from multiple modules (as
opposed to hard-coding constants), and also makes the output more
readable by skipping things we know for sure we don't want to convert
(as opposed to things we would like to but don't know _how_ to
convert).

Additionally, we export the constants from the luxi module too.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>