Change to using std::string instead of const char*

[brownierin]

Summary

This update should fix the use-after-free vulnerability reported in #37. While this wasn't exploitable, it's better to remove dangling pointers. We are now returning a string object instead of a pointer to a const char* that no longer exists after exiting the function.

Thank you for your report, @theHamsta!

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• [x] I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.

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