[wip] Sketch of how shared tasks might work

[njsmith]

See gh-266

This is surprisingly interesting and tricky.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 76 lines in your changes missing coverage. Please review.

Project coverage is 99.58999%. Comparing base (5ca9662) to head (d5327ca).

Files with missing lines	Patch %	Lines
src/trio/_shared_task.py	0.00000%	76 Missing ⚠️

Additional details and impacted files

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- Coverage   100.00000%   99.58999%   -0.41001%     
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  Files             124         125          +1     
  Lines           18460       18536         +76     
  Branches         1216        1224          +8     
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  Hits            18460       18460                 
- Misses              0          76         +76     

Files with missing lines	Coverage Δ
src/trio/_shared_task.py	0.00000% <0.00000%> (ø)

[njsmith]

One issue with this as currently written is that it uses outcome.unwrap() multiple times, which of course doesn't work.

We could mess around somehow trying to copy exceptions, but maybe it's better to do something simpler: if there's an error in a shared task, do raise SharedTaskError from <actual exception>. It's fine for multiple exceptions to have __cause__ pointing to the same object, because __cause__ doesn't get mutated.