Add fuzz tests for core pkg

[AnkushinDaniil]

fixes #868
Bugs found:

1. truncate
2. iter (resolved)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.45%. Comparing base (b240aba) to head (fe2ec1f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1961   +/-   ##
=======================================
  Coverage   78.45%   78.45%           
=======================================
  Files         100      100           
  Lines        9240     9240           
=======================================
  Hits         7249     7249           
  Misses       1353     1353           
  Partials      638      638           

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