ToBytes method
Implementation
void ToBytes(Uint8List s) {
var recip = FieldElement();
var x = FieldElement();
var y = FieldElement();
FeInvert(recip, Z);
FeMul(x, X, recip);
FeMul(y, Y, recip);
FeToBytes(s, y);
s[31] ^= FeIsNegative(x) << 7;
}