Add exception message for easier debugging with `Cancelled`

Occasionally we end up digging through logs trying to work out why something was cancelled, and eventually it occurred to me that Trio could help with this. I don't have a complete design proposal, but it would be great to handle each of three cases:

• a deadline was reached.
• a sibling task in the Nursery raised an exception, including the name of the erroring task
• someone called CancelScope.cancel(), including the name of the task in which this was called
  • add a new reason: str | None = None param, so that users can explicitly annotate why they cancelled something

I think we could represent this as a string argument to Cancelled like any other exception message; the only tricky part will be threading it through all the cancellation-delivery machinery

Comments and design input most welcome, of course!