Update codebase for newer PHPStan

[Firehed]

Newer PHPStan complains more rather than less about some GMP stuff. They're false-positives, see linked issues referenced in code.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.37%. Comparing base (971dbab) to head (2f8f8a9).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/BinaryString.php	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main      #99      +/-   ##
============================================
- Coverage     81.41%   81.37%   -0.04%     
  Complexity      335      335              
============================================
  Files            33       33              
  Lines          1033      988      -45     
============================================
- Hits            841      804      -37     
+ Misses          192      184       -8     

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