Add decoding of Luxi calls and unittests for LuxiOp

This patch adds a hand-coded decoder for LuxiCall arguments, as the
data-structure is not uniform enough for automated generation (even
for the serialisation, we had to add hints for some fields,
de-serialisation is even harder).

It also fixes a tiny issue with WaitForJobChange job ID encoding, and
adds unittests for the encoding/decoding of LuxiOp structures.

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