Add utility functions for list and dict operations

[Roaa421]

This file contains functions to remove duplicates from a list, count occurrences of items in a list, and reverse keys and values of a dictionary.

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Checklist

• I have read the [CONTRIBUTING]
• I have performed a self-review of my own code
• I have run the code locally and it works as expected
• I have commented my code, particularly in hard-to-understand areas

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