`LinkedTaskChannel.subscribe()`, explicit `asyncio` task cancel logging, `test_trioisms.py`

[goodboy]

Basically 2 parts:

• some better error logging and a somewhat involved scenario test that includes use of,
  • a sub-actor managed "service nursery" that can be used to service new async requests from a parent actor
  • broadcast channel usage for by multiple such "service tasks" within the sub-actor
  • ensuring errors in any service task cancels the others
  • making sure it all works with *infected asyncio` mode
• a new to_asyncio.LinkedTaskChannel.subscribe() which gives task-oriented broadcast functionality semantically equivalent to MsgStream.subscribe()
  • this makes it possible for multiple trio-side tasks to consume asyncio-side task msgs in tandem

This work was originally inspired as part of debugging a case in piker's service manager that ended up leading to https://github.com/python-trio/trio/issues/2258.

Included is a test that demonstrates the current trio behaviour in a new test_trioisms.py for tracking such situations in the future.

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TODO:

• [x] at least one test for the new LinkedTaskChannel.subscribe() support
• [x] nooz file

[goodboy]

Lul no idea but now windows wants to pass, k.