val kill : _ t ‑> unitkills t, which aborts all enqueued jobs that haven't started and all jobs enqueued
in the future. If t has already been killed, then calling kill t has no effect.
Note that kill does not effect currently running jobs in any way.
val is_dead : _ t ‑> boolis_dead t returns true if t was killed, either by kill or by an unhandled
exception in a job.