Add benchmarks for writeTxIn.

This commit adds a benchmark for the writeTxIn function.

bench_test.go

@@ -192,3 +192,12 @@ func BenchmarkReadTxIn(b *testing.B) {
 		btcwire.TstReadTxIn(bytes.NewBuffer(buf), 0, 0, &txIn)
 	}
 }
+
+// BenchmarkWriteTxIn performs a benchmark on how long it takes to write
+// a transaction input.
+func BenchmarkWriteTxIn(b *testing.B) {
+	txIn := blockOne.Transactions[0].TxIn[0]
+	for i := 0; i < b.N; i++ {
+		btcwire.TstWriteTxIn(ioutil.Discard, 0, 0, txIn)
+	}
+}

internal_test.go

@@ -132,3 +132,9 @@ func TstWriteTxOut(w io.Writer, pver uint32, version uint32, to *TxOut) error {
 func TstReadTxIn(r io.Reader, pver uint32, version uint32, ti *TxIn) error {
 	return readTxIn(r, pver, version, ti)
 }
+
+// TstWriteTxIn makes the internal writeTxIn function available to the test
+// package.
+func TstWriteTxIn(w io.Writer, pver uint32, version uint32, ti *TxIn) error {
+	return writeTxIn(w, pver, version, ti)
+}