Replace `readr::type_convert()` with `minty::type_convert()`?

[chainsawriot]

Hello. As the only function this package used from readr is type_convert(), it might be a good idea to switch to minty::type_convert(). More information about minty: https://github.com/gesistsa/minty

If you are interested, I can give you a PR.

Thank you very much!