Make Taproot spends standard + policy limits

This adds a `TxoutType::WITNESS_V1_TAPROOT` for P2TR outputs, and permits spending
them in standardness rules. No corresponding `CTxDestination` is added for it,
as that isn't needed until we want wallet integration. The taproot validation flags
are also enabled for mempool transactions, and standardness rules are added
(stack item size limit, no annexes).

src/script/sign.cpp

@@ -111,6 +111,7 @@ static bool SignStep(const SigningProvider& provider, const BaseSignatureCreator
     case TxoutType::NONSTANDARD:
     case TxoutType::NULL_DATA:
     case TxoutType::WITNESS_UNKNOWN:
+    case TxoutType::WITNESS_V1_TAPROOT:
         return false;
     case TxoutType::PUBKEY:
         if (!CreateSig(creator, sigdata, provider, sig, CPubKey(vSolutions[0]), scriptPubKey, sigversion)) return false;