Improve binary search tree implementation

[DaevMithran]

Changes

• Implement binary search tree without recursion

Checklist:

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

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.65079% with 4 lines in your changes missing coverage. Please review.

Project coverage is 95.06%. Comparing base (3481e51) to head (d3192c5). Report is 26 commits behind head on master.

Files with missing lines	Patch %	Lines
src/data_structures/binary_search_tree.rs	93.65%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #766 +/- ## ======================================= Coverage 95.05% 95.06% ======================================= Files 307 307 Lines 22731 22734 +3 ======================================= + Hits 21607 21611 +4 + Misses 1124 1123 -1 ```

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[github-actions[bot]]

This pull request has been automatically marked as abandoned because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[github-actions[bot]]

Please ping one of the maintainers once you commit the changes requested or make improvements on the code. If this is not the case and you need some help, feel free to ask for help in our Gitter channel. Thank you for your contributions!