Add Support for signatures on the G2 curve for BDN

[matejpavlovic]

Feel free to further edit the code in this PR, in particular adding proper tests.

[matejpavlovic]

I would also like to mention that my understanding of the code is still rather superficial. This shows, for example, that I did not manage to generalize the unit tests. Please make sure that the code actually works before merging. Thanks!

[Kubuxu]

Hey @AnomalRoil let me know what needs to get done to get this merged.

[AnomalRoil]

It needs more tests I guess, which has been on my to-do for like 2 months 😅

[Kubuxu]

While working on more tests, I discovered that hashToPoint is not implemented on bn256 G2 Point.
See the PR to upstream kyber: https://github.com/dedis/kyber/pull/428/files

This is not an issue with bls12381 we are using.

[Kubuxu]

Pull in Kubuxu@c5d6a11 for additional tests

[AnomalRoil]

I just need to add some retro-compat functions to avoid breaking public APIs and I'll release a new minor release with this 👍🏻