Musig2 key aggregation

[RCasatta]

super draft status

Wanted to try to implement https://github.com/ElementsProject/secp256k1-zkp/blob/master/src/modules/musig/musig-spec.mediawiki

Test vectors are currently passing, however, spec may change BlockstreamResearch/secp256k1-zkp#120 (comment)

[LLFourn]

Nice. I think this belongs in schnorr_fun eventually. This would be the first multi-party protocol in the *_fun crates and I have no idea what the API should be like so interested to know if you have some further ideas. I also need this by November for my own schemes!

[RCasatta]

I think this belongs in schnorr_fun eventually.

Moved into schnorr_fun and removed some TODOs, a little less drafty, however, spec may change BlockstreamResearch/secp256k1-zkp#120 (comment)

I have no idea what the API should be like so interested to know if you have some further ideas

At the moment I am trying to get a deeper understanding by playing around to have some ideas

I see BlockstreamResearch/secp256k1-zkp#131 (comment) is what Nicolas come up in C#

I also need this by November for my own schemes!

November is coming!

[LLFourn]

This is superseded by #80

Thanks for doing this @RCasatta. I used the test case you developed here for the new PR. If you have time to review, please let me know what you think of the API etc.