add new BytesNoWitness method to Block

This commit adds a new method to btcutil.Block for obtaining the
serialization of a Block *without* any witness data (if any).

block.go

@@ -34,6 +34,7 @@ func (e OutOfRangeError) Error() string {
 type Block struct {
 	msgBlock                 *wire.MsgBlock  // Underlying MsgBlock
 	serializedBlock          []byte          // Serialized bytes for the block
+	serializedBlockNoWitness []byte          // Serialized bytes for block w/o witness data
 	blockHash                *chainhash.Hash // Cached block hash
 	blockHeight              int32           // Height in the main block chain
 	transactions             []*Tx           // Transactions
@@ -68,6 +69,27 @@ func (b *Block) Bytes() ([]byte, error) {
 	return serializedBlock, nil
 }
 
+// BytesNoWitness returns the serialized bytes for the block with transactions
+// encoded without any witness data.
+func (b *Block) BytesNoWitness() ([]byte, error) {
+	// Return the cached serialized bytes if it has already been generated.
+	if len(b.serializedBlockNoWitness) != 0 {
+		return b.serializedBlockNoWitness, nil
+	}
+
+	// Serialize the MsgBlock.
+	var w bytes.Buffer
+	err := b.msgBlock.SerializeNoWitness(&w)
+	if err != nil {
+		return nil, err
+	}
+	serializedBlock := w.Bytes()
+
+	// Cache the serialized bytes and return them.
+	b.serializedBlockNoWitness = serializedBlock
+	return serializedBlock, nil
+}
+
 // Hash returns the block identifier hash for the Block.  This is equivalent to
 // calling BlockHash on the underlying wire.MsgBlock, however it caches the
 // result so subsequent calls are more efficient.