refactor: HexToOct

[alxkm]

Improving 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 46.44%. Comparing base (44d7cbb) to head (081d3e7).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5377      +/-   ##
============================================
+ Coverage     46.42%   46.44%   +0.01%     
- Complexity     2877     2878       +1     
============================================
  Files           525      525              
  Lines         15406    15400       -6     
  Branches       2916     2916              
============================================
  Hits           7153     7153              
+ Misses         7954     7947       -7     
- Partials        299      300       +1     

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