Cleanup perfect_cube.rs

[vil02]

Description

There are few problems with perfect_cube.rs:

• the perfect_cube is not tested (note the typo in the test_perfect_cube),
• perfect_cube and perfect_cube_binary_search should have the same input type,
• perfect_cube does not give the correct result for 64 = 4*4*4,
• I do not see any reason why to exclude negative numbers from being perfect cubes. For example -8 = (-2)*(-2)*(-2), so why -8 is not a perfect cube here?

This PR fixes above issues by removing perfect_cube function and reorganizes the tests.

Type of change

Please delete options that are not relevant.

• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I ran bellow commands using the latest version of rust nightly.
• I ran cargo clippy --all -- -D warnings just before my last commit and fixed any issue that was found.
• I ran cargo fmt just before my last commit.
• I ran cargo test just before my last commit and all tests passed.
• I added my removed algorithm to from the corresponding mod.rs file within its own folder, and in any parent folder(s).
• I checked COUNTRIBUTING.md and my code follows its guidelines.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (cb3b21d) 94.57% compared to head (647cdbf) 94.60%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #655      +/-   ##
==========================================
+ Coverage   94.57%   94.60%   +0.03%     
==========================================
  Files         285      286       +1     
  Lines       22906    22924      +18     
==========================================
+ Hits        21664    21688      +24     
+ Misses       1242     1236       -6     

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