Feature Request: add ability to get x smallest or largest from queryabletree

[amazingvince]

I am currently learning rust so I dont think I can do it but looking at the code should not be too bad. I would expect return Vec first value being either the min or max depending on call. Might make a PR is I can figure it out. Still learning the basics.

[Leo-LiHao]

Thank you! This repo is a part of our course project which helps us to learn Rust. Contribution welcome. Happy coding.