Bip340 support

[GambolingPangolin]

This change adds bindings to the schnorr signing functionality in libsecp256k1, but behind a flag. The versions of libsecp256k1 that ship with popular linux distros do not have this functionality enabled. However, nix users can add the following shell.nix to explore the changes:

let nixpkgs = import <nixpkgs> {};
in

with nixpkgs.pkgs; mkShell {
  buildInputs = [
    cabal-install
    ghc
    gmp
    pkg-config
    secp256k1
  ];
}

As long as they are using a recent version of nixpkgs.

[ysangkok]

I previously added this in #26 but it was removed in 0f862c2. But maybe things have changed, I definitely think the library should include this. I'd encourage you to fork the library on Hackage if BIP-340 is still deemed "non-standard".

[GambolingPangolin]

There has been some discussion about whether to include this now that taproot is active over in haskoin-core and I think we reached consensus that we should include it. It looks like your patch is a bit more comprehensive, so I'm gonna try resurrecting it and changing my patch for taproot signing to use your API.

[ysangkok]

Note that my patch was for a pre-release version of secp256k1. It might still be resurrectable, but I wanted to mention that.

[ysangkok]

@jprupp Can this be merged now that Ubuntu 24.04 is out and all target distributions have schnorr support?