Currently, the Status that's used to cancel an operation with via CancellationSource is entirely up to the cancellation logic. This makes it needlessly onerous for cancellation logic to accept a Status with which to cancel the operation from the calling code that requests cancellation. We should change the signature of CancellationSource::cancel() to accept (perhaps optionally) a Status so that callers can directly provide the Status to the cancellation logic, as opposed to through some contrived handoff through external state.