Think about how MiniscriptKey interacts with Musig keys

[sanket1729]

Integrating musig2 keys with the existing API introduces a couple of new challenges:

1. We currently support Terminal::Pk(Pk) in our miniscript decoding. This means we should parse musig(A,B,C) as a single Pk.
2. At the same time, we want the translation API to allow to translation of native A, B and C.

Seems like we might to re-design some parts of the codebase to support it. cc @apoelstra

[sanket1729]

It seems like the best solution would be to support parsing as a new struct KeyExpr.

struct KeyExpr<Pk: MiniscriptKey> {
      SingleKey(Pk),
      Musig(Vec<KeyExpr<Pk>>),
}

1. Then we change Terminal::Pk(Pk) => Terminal::Pk(KeyExpr).
2. For Pkh, as discussed elsewhere first, we need to change from Pkh(Pk::Hash) to Pkh(Option, Pk::Hash). That will changed to Pkh(Option<KeyExpr<Pk>>, Pk::Hash)
3. Multi will change to use KeyExpr as above

[apoelstra]

This sounds like the right direction to me. The keyhash stuff is a bit ugly but I think that's inherent to the problem domain.