Switch Luxi TH code from simple to custom fields

This is needed so that we have more flexibility in generating Luxi
serialisation code (deserialisation is still custom). Also, only
exceptions are now using the 'simple' field types, so we might be able
later to convert and remove that TH code as well.

Since we will use custom serialisation fields in the future, we change
the order of serialisation for custom-save fields; Luxi uses
positional as opposed to name-based ordering, so we need to keep this
stable.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>