Adding `sel` and `libsodium-bindings`

[Kleidukos]

Hi @tomsmeding! Would it be possible to add sel and libsodium-bindings?

[tomsmeding]

Do you intend to use availability of these packages for something specific? I haven't heard of these packages before (which is not a claim against their quality, by the way), and I'd prefer to limit new packages to those that are either helpful to a large enough number of people (by being well-known and hence possibly "expected" to be there) or useful specifically for some other reason (as was the intent for Copilot here -- not sure if that ever got anywhere).

[Kleidukos]

@tomsmeding yeah, I wanted to share a snippet that makes use of constant-time bytestring comparison utilities provided by libsodium. :)