enabling zeroize for field elements

[zhenfeizhang]

This PR enables zeroize for field elements -- allows for better memory safety for upper layer libs such as pairing or bls signatures. Would love to see this functionality merged to the main repo.

[str4d]

Hi! It looks like this branch was modified when you published a separate crate, because the PR is from your fork's master branch. Please either rebase this branch to fix it, or make a new PR with a change-specific branch.

Separately, I'm not sure we want to add a required Zeroize bound, but instead have a zeroize feature and some trait that downstream users can add to their bounds, to enforce that a particular type must be zeroizable (which can then be applied to e.g. secret scalars, and not applied to field elements used as coordinates for public points). In any case, PrimeFieldRepr is in the process of being eliminated, in favour of having PrimeField::Repr be the encoded type (see zcash/librustzcash#223).

[zhenfeizhang]

Thanks! I have created another PR: #24