Auto-define a LuxiReq data type

We currently auto-generate a LuxiOp data type, which holds the entire
operation (including parameters). However, having a data type just for
the method call would be useful, so let's change THH to also
defineSADT for the Luxi constructors.

Currently I don't know how to match automatically a LuxiReq to its
LuxiOp counterpart (not even sure we need that), so any matching will
remain manual.

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