Support Zeroize

[afck]

Cryptographic applications should be able to wipe sensitive data from memory before deallocating it. E.g. the elements of Fr can be used as secret BLS keys.

Without support from the bls12_381 crate itself, any attempt to clear such data necessarily depends on an implementation detail of bls12_381 that is not part of the public API, e.g. Fr not using any pointers internally.

The safest way to support this would be to add explicit methods for clearing, or to impl Zeroize for Fr. If the added zeroize dependency is a concern, it could be made optional, and put behind a feature flag.

[ebfull]

Thanks! I love zeroize and definitely think we should support it.