Time.TimeoutTimeout values.
val v : _ Mono.t -> Mtime.Span.t -> tv 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.
val none : tnone is an infinite timeout.
run t fn runs fn () but cancels it if it takes longer than allowed by timeout t.