SIGHASH_ANYPREVOUT for Taproot Scripts.

Would add a type of signature that isn't tied to one exact coin, so one pre-signed payment can respond to many possible ones.

Systems that settle later on the blockchain often prepare a reply payment in advance. Today each signature commits to one exact coin, so a new signature is needed for every case. This opt-in signature, for taproot scripts, could be reused for any coin with a compatible script.