keys/hd/address.go
func addScalars(a []byte, b []byte) []byte {
	aInt := new(big.Int).SetBytes(a)
	bInt := new(big.Int).SetBytes(b)
	sInt := new(big.Int).Add(aInt, bInt)
	x := sInt.Mod(sInt, btcec.S256().N).Bytes()
	x2 := [32]byte{}
	copy(x2[32-len(x):], x)
	return x2[:]
}
func addScalars(a []byte, b []byte) []byte {
	aInt := new(big.Int).SetBytes(a)
	bInt := new(big.Int).SetBytes(b)
	sInt := new(big.Int).Add(aInt, bInt)
	x := sInt.Mod(sInt, btcec.S256().N).Bytes()
	x2 := [32]byte{}
	copy(x2[32-len(x):], x)
	return x2[:]
}
// modular big endian addition