Replace libsodium dependency

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](https://docs.rs/blake2/latest/blake2)

Inspect the crate's current libsodium bindings and identify the scalar operations it actually uses. Read the linked Blake2 library documentation before assessing the hashing substitution. Done means the crate no longer requires the C library or unsafe bindings while preserving its existing signcryption behavior.

Written by the indexing model from the issue text.