feat: The Arity.of(int, int) return NAry when arguments are the same.

[nstdio]

No description provided.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.87%. Comparing base (d58e7d9) to head (30ef3e9).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #91   +/-   ##
=========================================
  Coverage     94.87%   94.87%           
- Complexity      174      175    +1     
=========================================
  Files            20       20           
  Lines           351      351           
  Branches         40       40           
=========================================
  Hits            333      333           
  Misses           13       13           
  Partials          5        5           

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