Draft BIP for OP_TXHASH and OP_CHECKTXHASHVERIFY

This is partly because TxhashKey does not encompass the additional uses of hashes that TXHASH+CSFS does, which have unresolved technical objections. There's an acknowledgment of not being fully informed about the criticisms against CSFS, but there's confidence in addressing the technical concerns related to TxhashKey.

Furthermore, the conversation touches upon reservations regarding pre-defined modes due to observed challenges in similar scenarios, such as with APO/APOAS. Despite these reservations, there's an expressed difficulty in identifying downsides to offering some best effort pre-defined modes in addition to a complete set of flags. This suggests a willingness to explore the balance between flexibility and guidance in the implementation of these systems.