io: simplify WriteBytes()

Which speeds it up at least twofold for a typical 32-bytes write (and that's
for a very naïve test that allocates new BufBinWriter on every iteration):

pkg: github.com/CityOfZion/neo-go/pkg/io
BenchmarkWriteBytes-8           10000000               124 ns/op
BenchmarkWriteBytesOld-8         5000000               251 ns/op

pkg/io/binaryrw_test.go

@@ -210,6 +210,11 @@ func TestWriteBytes(t *testing.T) {
 	buf := bw.Bytes()
 	assert.Equal(t, 4, len(buf))
 	assert.Equal(t, byte(0xde), buf[0])
+
+	bw = NewBufBinWriter()
+	bw.Err = errors.New("smth bad")
+	bw.WriteBytes(bin)
+	assert.Equal(t, 0, bw.Len())
 }
 
 type testSerializable uint16