Make BLS impl compatible with IRTF doc

[LLFourn]

irtf here: https://github.com/cfrg/draft-irtf-cfrg-bls-signature

I dunno what has to be done here. One decision we need to make is whether we want minimal-pubkey-size or minimal-signature-size. I think I implemented it with the minimal-pubkey-size variant. Otherwise I think the scheme I implemented is compatible with the irtf but we should check this with some test vectors. I don't think I even implemented verify yet.

Note i think we will need our own implementation rather than relying on 3rd party libraries since we will use the signatures is some non-black-box ways.