In preparation for the potential Taproot soft fork, libconsensus should be updated to support verifying Taproot scripts.

There are two changes I see as necessary:

- Add a VERIFY_TAPROOT flag
- Change the API to support passing in spent outputs (bitcoinconsensus_verify_script_with_spent_outputs?)
#include <script/bitcoinconsensus.h>
#include <primitives/transaction.h>
#include <pubkey.h>
#include <script/interpreter.h>
#include <version.h>
namespace {
class TxInputStream