compress method
Implementation
CompressedEdwardsY compress() {
final FieldElement r = _z.invert();
final FieldElement x = _x * r;
final FieldElement y = _y * r;
final List<int> s = y.toByteArray();
s[31] |= x.isNegative() << 7;
return CompressedEdwardsY(s);
}