Taproot-native (Re)bindable Transactions.

Would switch on three script commands together, OP_TEMPLATEHASH, OP_CHECKSIGFROMSTACK and OP_INTERNALKEY, to improve layer-two systems.

These small additions to Taproot scripts would let coins commit to the payment that spends them, check signatures on any message, and reuse the Taproot key cheaply. Together they make 'rebindable' signatures possible, enabling a simpler kind of payment channel called LN-Symmetry (Eltoo). How it would be activated is left for later.