Use an up-to-date libsodium

[darosior]

We use libsodium, a vouched crypto library, as our Noise backend.

The Rust wrapper we are using, sodiumoxide will no longer update libsodium unless this is a security update.

Maybe we could migrate to another resolver for snow, but ring isn't an option and the default resolver brings many more dependencies (30 more!). Maybe we could also eventually upstream a resolver which uses the rust wrapper of libhydrogen.
An alternative is to use an updated wrapper for libsodium, which the author maintains in a fork: https://github.com/jedisct1/libsodium-sys-stable.