General refactor

[Robzz]

So, this repo started pretty much as an afternoon experiment. Then it grew a little. It's at the point where the code looks pretty damn terrible, so next order of business before implementing any new non trivial features should be to clean it up.

[Robzz]

#37 did a good bit to clean up the code, but there's still some more things I'd like to do :

• derive_more is barely used anymore, the dependency can probably be dropped at little cost.
• Remove travis related stuff since I switched to Github actions
• See if switching from snafu to anyhow makes anything simpler.