Session::run_async

[decahedron1]

Natively asynchronous Session::run.

TODO

• Check unsoundness. This code is probably very terrible. Anyone able to review? :^)
• Make the example more functional
• Maybe remove run_with_options_async and just always create a RunOptions that we share with the future, so it can automatically cancel when it drops.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 98 lines in your changes are missing coverage. Please review.

Project coverage is 43.20%. Comparing base (39bd0d0) to head (8ba1b73).
Report is 4 commits behind head on main.

Files	Patch %	Lines
src/session/async.rs	0.00%	55 Missing ⚠️
src/session/mod.rs	0.00%	43 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #174      +/-   ##
==========================================
- Coverage   46.07%   43.20%   -2.87%     
==========================================
  Files          23       24       +1     
  Lines        1476     1574      +98     
==========================================
  Hits          680      680              
- Misses        796      894      +98     

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