Tor support & x25519 encryption

[scilio]

While adding in tor support, I realized we would be better off using x25519 encryption. This allows us to use a single public key for encryption and onion address, the same way we do for slatepacks.

[phyro]

Thanks for addressing the comments @scilio :+1: I added a minor comment to the issue you opened that will help people understand the design from the readme