move any schnorr logic into nostr-rs

[futurepaul]

I see there's a few places where we're using types from secp256k1. that should probably be re-exported from nostr-rs. ideally an "application author" doesn't have to think about that stuff.

there might be places where the whole schnorr logic probably belongs in nostr-rs, like peer_pk = PublicKey::from_str(ev.tags[0].content()).unwrap(); feels like nostr-rs's job

[emeceve]

I would suggest re-export secp256k1 in first place but I think the Keys struct can be used instead