refactor: HashMap

[alxkm]

Class cleanup.
Readability improvements.
Added 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 Uncyclopedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 50.65%. Comparing base (6b7a1fd) to head (8c3ea5a).

Files with missing lines	Patch %	Lines
...rithms/datastructures/hashmap/hashing/HashMap.java	90.00%	1 Missing and 5 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5426      +/-   ##
============================================
+ Coverage     50.17%   50.65%   +0.48%     
- Complexity     3121     3131      +10     
============================================
  Files           524      523       -1     
  Lines         15143    15125      -18     
  Branches       2878     2879       +1     
============================================
+ Hits           7598     7662      +64     
+ Misses         7229     7141      -88     
- Partials        316      322       +6     

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