Port to libsodium

It looks like sodiumoxide only depends on std for serializing with serde, disabling default features should preserve no-std for the library.

As I more or less don't know what I'm doing with crypto stuff, there's probably some bits of code in here that are super naive. Also it hasn't gone through much testing, so I don't know what the value is there either.

I'd like to integrate pkgar-keys at some point (little project I've been working on to store keys for packaging and verification), but I'd appreciate some guidance on how those should be formatted. More discussion should probably happen in the repo: https://gitlab.redox-os.org/MggMuggins/pkgar-keys