Simplify function context and arity validation

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.90%. Comparing base (ae472a8) to head (e551022).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/function.rs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #178      +/-   ##
==========================================
- Coverage   82.97%   82.90%   -0.08%     
==========================================
  Files          70       70              
  Lines        3577     3562      -15     
==========================================
- Hits         2968     2953      -15     
  Misses        609      609              

Flag	Coverage Δ
unit	82.90% <94.44%> (-0.08%)	⬇️

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