Implement Taproot signature hashing (BIP 341)

This implements the new sighashing scheme from BIP341, with all relevant
whole-transaction values precomputed once and cached.

Includes changes to PrecomputedTransactionData by Pieter Wuille.

src/validation.cpp

@@ -1538,7 +1538,7 @@ bool CheckInputScripts(const CTransaction& tx, TxValidationState &state, const C
         return true;
     }
 
-    if (!txdata.m_ready) {
+    if (!txdata.m_spent_outputs_ready) {
         std::vector<CTxOut> spent_outputs;
         spent_outputs.reserve(tx.vin.size());