Port lemmas from model/schedule/uni/schedule.v

- port completed_implies_scheduled_before
- port lemmas on service prior to arrival
- port scheduled_implies_pending and greatly simplify the proof while at it
- port and simplify job_pending_at_arrival
- port cumulative_service_implies_scheduled and simplify proof of
  positive_service_implies_scheduled_before
- port service_is_a_step_function