refactor: Bag data structure

[alxkm]

Improving code readability.
Adding tests.

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 43.28%. Comparing base (e8985b3) to head (587346e).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5340      +/-   ##
============================================
+ Coverage     43.06%   43.28%   +0.21%     
- Complexity     2672     2682      +10     
============================================
  Files           522      522              
  Lines         15469    15456      -13     
  Branches       2953     2952       -1     
============================================
+ Hits           6662     6690      +28     
+ Misses         8521     8481      -40     
+ Partials        286      285       -1     

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