Add support for a single level form of subsidiary deadlines + uninterruptibility in Interruptible with a syntax that looks like:

future.get(interruptible->makeTimer(deadline, code));

That syntax should produce a type convertible to an Interruptible* which wraps underlying calls to checkForInterrupt or waitForConditionOrInterruptUntil calls in the appropriate guards. It should replace the more wordy:

interruptible->runWithDeadline(deadline, code, [&]{
  future.get(interruptible);
});

when a subsidiary deadline or uninterruptible invocation is needed at the very bottom of the callstack.

That type should not nest and should not alter the state of the parent interruptible