Time.TimeoutSourceTimeout values.
v clock duration is a timeout of duration, as measured by clock. Internally, this is just the tuple (clock, duration).
seconds clock duration is a timeout of duration seconds, as measured by clock.
run t fn runs fn () but cancels it if it takes longer than allowed by timeout t.
run_exn t fn runs fn () but cancels it if it takes longer than allowed by timeout t, raising exception Timeout.