Some unit tests improvements

In preparation for unit-testing the Luxi definitions, this improves
the auto-generation of OpCodes: currently this uses unbounded plain
Strings for names and fields, and this creates too big test sizes
(e.g. when automatically generating job sets).

Furthermore, there are some more cleanups: switching from liftM to
applicative style for easier definition, and some improvements in
generators.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>