loci: add transparent union for list iteration

This is the first step in getting rid of both inheritance unions and header
classes. Callers can now use either e.g. of_action_t or of_object_t for list
iteration. Once all callers are converted to of_object_t the union will be
removed.
loci: convert internal users away from inheritance union

This mostly affects list accessors.
loci: convert locitest away from inheritance union
Merge into master from pull request #342:
loci: begin removing inheritance unions (https://github.com/floodlight/loxigen/pull/342)