idea: an `ActorCancelled(ContextCancelled)` exc type for better OoB cancelled handling

[goodboy]

Per recent testing effort, issue-docs drafting and generally ranting in the trio matrix room 🫣 here is the recent content driving this idea,

• #399 starts dedicated testing around such scenarios
• #400 attempts to justify the need for more then just trio.Cancelled
• #391 demos why trio.Cancelled and so called "exception masking" can be problematic in a distributed system where error/cancel relay is important for distributed-sys feats.

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New supporting feats in trio we should try here

With release 0.31.0 we get a new Cancelled.reason: str which can be set at the .cancel(reason) call,

• introducing patch, https://github.com/python-trio/trio/issues/3232
• docs on CancelScope.cancel()
• low level impl details,
  • internal CancelReason struct
  • which is now a private instance var on all scopes.
  • how and where it's set in `._cancel()
  • example low-level-internal usage in case we wanted to overload the type in ._child_finished()