Go: preparation for support generics

[masatake]

#4397 becomes too big.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.04%. Comparing base (72c69ba) to head (471782f).
⚠️ Report is 21 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4401   +/-   ##
=======================================
  Coverage   86.04%   86.04%           
=======================================
  Files         254      254           
  Lines       63762    63778   +16     
=======================================
+ Hits        54862    54878   +16     
  Misses       8900     8900           

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