Unspendable keys in descriptors

[andrewtoth]

@salvatoshi

Finally, a wallet policy might have the same @i multiple times (especially on taproot); in your version you’d either be concatenating the corresponding pubkey multiple times, or would need a stateful parser to skip the ones that have been seen already.

If we would like an unspendable internal xpub to be standardized between descriptors and wallet policies, then we must remove duplicates. I think sorting and removing duplicates of all xpubs, then concatenating and SHA256 hashing the result would be the simplest to implement.

We would also have to restrict all xpubs to xpub/<0;1>/* with no allowance for optional derivation paths. I think that’s ok because that allowance is for legacy wallets, who would not have taproot support anyways.