add new WitnessHash method to Tx

This commit adds a new method to btcutil.Tx which implements a memoized
equivalent to the `Hash` method but for the newly defined `wtxid.

tx.go

@@ -24,6 +24,7 @@ const TxIndexUnknown = -1
 type Tx struct {
 	msgTx         *wire.MsgTx     // Underlying MsgTx
 	txHash        *chainhash.Hash // Cached transaction hash
+	txHashWitness *chainhash.Hash // Cached transaction witness hash
 	txIndex       int             // Position within a block or TxIndexUnknown
 }
 
@@ -48,6 +49,21 @@ func (t *Tx) Hash() *chainhash.Hash {
 	return &hash
 }
 
+// WitnessHash returns the witness hash (wtxid) of the transaction.  This is
+// equivalent to calling WitnessHash on the underlying wire.MsgTx, however it
+// caches the result so subsequent calls are more efficient.
+func (t *Tx) WitnessHash() *chainhash.Hash {
+	// Return the cached hash if it has already been generated.
+	if t.txHashWitness != nil {
+		return t.txHashWitness
+	}
+
+	// Cache the hash and return it.
+	hash := t.msgTx.WitnessHash()
+	t.txHashWitness = &hash
+	return &hash
+}
+
 // Index returns the saved index of the transaction within a block.  This value
 // will be TxIndexUnknown if it hasn't already explicitly been set.
 func (t *Tx) Index() int {