Implements event-driven promises capability through the aio callback mechanism

[shikokuchuo]

Thanks @jcheng5 for the prototype in #28 .

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 10.16949% with 53 lines in your changes are missing coverage. Please review.

Project coverage is 88.06%. Comparing base (b08f27a) to head (b8ed236).
Report is 6 commits behind head on main.

❗ Current head b8ed236 differs from pull request most recent head e8226fb. Consider uploading reports for the commit e8226fb to get more accurate results

Files	Patch %	Lines
src/aio.c	10.41%	43 Missing ⚠️
R/context.R	0.00%	8 Missing ⚠️
src/later_shim.cpp	0.00%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #29      +/-   ##
==========================================
- Coverage   89.55%   88.06%   -1.49%     
==========================================
  Files          23       24       +1     
  Lines        3139     3193      +54     
==========================================
+ Hits         2811     2812       +1     
- Misses        328      381      +53     

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