Allow accessing signature::UnparsedPublicKey bytes

[tdryer]

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 ~and algorithm method~ for signature::UnparsedPublicKey.

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