OP_CAT in Tapscript.

Defines OP_CAT, a Taproot script instruction that joins two pieces of data into one, to be added by soft fork.

Taproot scripts have no general way to combine pieces of data, which limits what they can do. OP_CAT would fill that gap, with a cap of 520 bytes on the result, making it possible to build and check structures such as Merkle trees inside scripts. It reuses the number of Bitcoin's original OP_CAT instruction.