txscript: Make executeOpcode take opcode and data.
txscript: Make executeOpcode take opcode and data.

This converts the executeOpcode function defined on the engine to accept
an opcode and data slice instead of a parsed opcode as a step towards
removing the parsed opcode struct and associated supporting code altogether.

It also updates all callers accordingly.

ltcsuite/ltcd,ltcsuite/ltcd,tb00/btcd,davecgh/btcd,davecgh/btcd,tb00/btcd,btcsuite/btcd,btcsuite/btcd