Replace libsodium dependency

[mberry]

There's a fair bit of unsafe bindings and a C lib requirement to use the crate, would be nice to remove that and have it in pure rust.

We only need a few scalar operations, hashing can substituted with the Blake2 library

[Artoria2e5]

The curve25519-dalek people probably have these scalars covered. (There's always dryoc too!)