Make Printer a final class

[Baptouuuu]

Fix #70

[codecov]

Codecov Report

❌ Patch coverage is 98.47716% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.88%. Comparing base (0edc5bb) to head (3f90289).
⚠️ Report is 6 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/Runner/Printer/Proof.php	97.27%	3 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop      #97      +/-   ##
=============================================
+ Coverage      91.86%   91.88%   +0.02%     
+ Complexity      2244     2223      -21     
=============================================
  Files            151      151              
  Lines           9443     9455      +12     
=============================================
+ Hits            8675     8688      +13     
+ Misses           768      767       -1     

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