Add PSBT::CacheUnsignedTxPieces

Fetches the PSBTv2 fields from PSBTv0's global unsigned tx. This allows
us to pretend everything internally is a PSBTv2 and makes things easier
to work with.

src/psbt.cpp

@@ -398,3 +398,36 @@ uint32_t PartiallySignedTransaction::GetVersion() const
     }
     return 0;
 }
+
+void PartiallySignedTransaction::SetupFromTx(const CMutableTransaction& tx)
+{
+    tx_version = tx.nVersion;
+    fallback_locktime = tx.nLockTime;
+
+    uint32_t i;
+    for (i = 0; i < tx.vin.size(); ++i) {
+        PSBTInput& input = inputs.at(i);
+        const CTxIn& txin = tx.vin.at(i);
+
+        input.prev_txid = txin.prevout.hash;
+        input.prev_out = txin.prevout.n;
+        input.sequence = txin.nSequence;
+    }
+
+    for (i = 0; i < tx.vout.size(); ++i) {
+        PSBTOutput& output = outputs.at(i);
+        const CTxOut& txout = tx.vout.at(i);
+
+        output.amount = txout.nValue.GetAmount();
+        output.script = txout.scriptPubKey;
+    }
+}
+
+void PartiallySignedTransaction::CacheUnsignedTxPieces()
+{
+    // To make things easier, we split up the global unsigned transaction
+    // and use the PSBTv2 fields for PSBTv0.
+    if (tx != nullopt) {
+        SetupFromTx(*tx);
+    }
+}

src/psbt.h

@@ -738,6 +738,8 @@ struct PartiallySignedTransaction
     [[nodiscard]] bool Merge(const PartiallySignedTransaction& psbt);
     bool AddInput(const CTxIn& txin, PSBTInput& psbtin);
     bool AddOutput(const CTxOut& txout, const PSBTOutput& psbtout);
+    void SetupFromTx(const CMutableTransaction& tx);
+    void CacheUnsignedTxPieces();
     PartiallySignedTransaction() {}
     explicit PartiallySignedTransaction(const CMutableTransaction& tx);
     /**