Bump dependencies; apply select clippy recommendations

[jqnatividad]

Hi @YS-L ,

As I now recommend pairing qsv with csvlens, I took the liberty of making some changes

• use Rust 2021 edition
• bump dependencies to latest minor version
• apply select clippy recommendations
• implemented some TODOs

Let me know if you want me to squash the commits and/or remove some clippy lint recommendations.

Thanks!