Use Public/SecretKey for the elliptic-curve implementation

[daxpedda]

Currently we are using ProjectivePoint and Scalar directly instead of PublicKey and SecretKey respectively.

The reason was that curve25519_dalek::Scalar supports Zeroize but not ZeroizeOnDrop.
But elliptic_curve::SecretKey supports ZeroizeOnDrop but not Zeroize.
(this was one of the main reasons I worked on introducing ZeroizeOnDrop in the first place)

The best solution here would be to introduce a SecretKey for Ristretto255, which currently doesn't exist, we could introduce this type ourselves in opaque-ke though.

Not sure what to do about elliptic_curve::PublicKey, as it implements neither. Is not zeroing out the public key acceptable?

This is analogous to facebook/voprf#57 and is mainly for code improvement and cleaner deserialization, it doesn't actually change anything.

[kevinlewi]

Hmm, I see. Yes, not zeroing out the public key should be fine in this case.