Update dependencies, move `approx` to dev dependency and edition 2021

[remi-dupre]

Hi! Just lurking on your crate and saw a bit of maintenance I could suggest:

1. move approx to dev dependencies, as it is only used for tests (it's a single dependency crate now, congrats !)
2. update dependencies (cargo outdated now report a full up to date tree)
3. update project to edition 2021, no direct value here, it may be easier on future contributors and just required to update a few module imports :shrug:

[nathan-osman]

Thanks so much!