cleanup: use PublicKey, SecretKey, Signature from wrapped crypto module

[okdistribute]

Checklist

• tests pass
• tests and/or benchmarks are included
• documentation is changed or added

Context

Instead of using the ed25519_dalek directly in feed.rs, import it from the crypto wrapper. See #6

This is still forwarding the type though; to fully complete the task this issue calls for wrapping the type as well.

Semver Changes

Patch

[bltavares]

Thank you! Merged and released on the beta versions