[wip] Sketch of how shared tasks might work

[njsmith]

See gh-266

This is surprisingly interesting and tricky.

[codecov]

Codecov Report

Merging #303 (360855d) into master (1c15500) will decrease coverage by 0.37%.
The diff coverage is 0.00%.

Additional details and impacted files

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##           master     #303      +/-   ##
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- Coverage   98.94%   98.58%   -0.37%     
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  Files         113      114       +1     
  Lines       16919    16982      +63     
  Branches     3050     3064      +14     
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  Hits        16741    16741              
- Misses        123      186      +63     
  Partials       55       55              

Files Changed	Coverage Δ
trio/_shared_task.py	0.00% <0.00%> (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.