should add musig() combinator for musig-combining keys

[apoelstra]

after taproot :)

[sanket1729]

Party being addressed in #433. The spec seems to be solid now. I think we are ready to go on this front now.

[bigspider]

In case any good soul wants to bring this to the finish line, just linking to the specs for musig() key expressions: BIP-390

[starius]

I'm adding musig() expression support in #954