Implement `AsRef<[u8]>` for `signature::UnparsedPublicKey`, making it
possible to access the bytes of the public key. For consistency, I did
the same for `agreement::UnparsedPublicKey`, although it already has a
`bytes` method. `agreement::PublicKey` already uses the `AsRef<[u8]>`
approach.
Also, add missing `Debug` implementation for
`signature::UnparsedPublicKey`.
I agree to license my contributions to each file under the terms given
at the top of each file I changed.
2afc921340