make async optional

[s5suzuki]

• rename from Link to AsyncLink
• make async optional
• speed up github actions build

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.09%. Comparing base (cd6a0ba) to head (58a1a29).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #189      +/-   ##
===========================================
- Coverage   100.00%   99.09%   -0.91%     
===========================================
  Files          160      165       +5     
  Lines        11593    12174     +581     
===========================================
+ Hits         11593    12064     +471     
- Misses           0      110     +110     

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