Replace asyncio with concurrent.futures, add tests

[ThomasPluck]

Resolves:

• https://github.com/dan-fritchman/Hdl21/issues/122

Twinned with:

• https://github.com/Vlsir/Vlsir/pull/68

Major changes:

• Scrub asyncio
• Deprecate sim.run_async
• Add test to test multi-input simulation running in parallel
• Add test to demonstrate solution to: https://github.com/erdewit/nest_asyncio/issues/57

[codecov[bot]]

Codecov Report

Merging #143 (a26fc1c) into main (2e9f5e9) will increase coverage by 0.28%. The diff coverage is 96.15%.

@@            Coverage Diff             @@
##             main     #143      +/-   ##
==========================================
+ Coverage   87.55%   87.84%   +0.28%     
==========================================
  Files          95       95              
  Lines        8559     8737     +178     
==========================================
+ Hits         7494     7675     +181     
+ Misses       1065     1062       -3     

Impacted Files	Coverage Δ
hdl21/sim/data.py	91.63% <66.66%> (-0.90%)	:arrow_down:
hdl21/sim/tests/test_sim.py	97.35% <100.00%> (+0.24%)	:arrow_up:

... and 7 files with indirect coverage changes

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[dan-fritchman]

Love it, ship it!