zkp: Musig2 module

[GeneFerneau]

Adds high-level Rust bindings for Musig2 multi-signature protocol

[GeneFerneau]

Diff looks much larger than it actually is, since I pulled in @jonasnick's changes to secp256k1-zkp.

Main changes are in secp256k1-zkp-sys/src/zkp.rs and rust-secp256k1-zkp/src/zkp/musig.rs.

Will rebase, and update as changes are merged from #131

[GeneFerneau]

Updated on #131/0c43aae

[GeneFerneau]

Rebased on latest master ab7f6d8

Updated to use schnorrsig::PublicKey.

Still need to rebase on @jonasnick's latest work, but may wait until the work is finished in their PR.

[elsirion]

Still need to rebase on @jonasnick's latest work, but may wait until the work is finished in their PR.

Agreed, too much work otherwise. Thanks for writing the code in the first place, before the PR was even merged! Was super happy I didn't have to do it myself 😃

[sanket1729]

Hi @GeneFerneau, musig2 spec and all PRs(but the tweaking support) have been merged upstream. Do you have any plans to resume this?

[sanket1729]

Hi @GeneFerneau, I could not resist playing with this branch. I made a few fixups to make it work after #46. Your previous work was very clean, this only required a few changes: https://github.com/sanket1729/rust-secp256k1-zkp/tree/pr29

If you are busy with other commitments, I can pick this up. Otherwise, feel free to cherry-pick and squash above PR and lets get to review :)