Add NoDiscard attribute on mutation free methods and the ones return Attempt<SideEffect>

[Baptouuuu]

Closes #21

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.47%. Comparing base (bdeec74) to head (b2ac1b5).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop      #24   +/-   ##
==========================================
  Coverage      97.47%   97.47%           
  Complexity       429      429           
==========================================
  Files             68       68           
  Lines           2728     2728           
==========================================
  Hits            2659     2659           
  Misses            69       69           

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