Add message with debugging info to Cancelled

[jakkdl]

fixes #3232 by adding a CancelReason that's tracked by both CancelScope and CancelStatus and then used when constructing a Cancelled to be raised.

Remaining stuff:

• I'm not 100% sure that both CancelScope and CancelStatus needs to have a CancelReason member - initially only CancelStatus had it but that wasn't enough, but I haven't tried to see if I can get away with only CancelScope having it.
  • It's now only tracked in CancelScope
• I also found a bug as I was reviewing and adding a final test, where manually setting cs.deadline=-inf and then checkpointing somehow ends up as an explicit cancelled.
• from_thread_check_cancelled needs some special handling
  • I managed to fix this.. but in a way that created cyclic garbage. So need a better fix