Function elligator_encode 

pub(crate) fn elligator_encode(
    u: &FieldElement51,
) -> (FieldElement51, FieldElement51, FieldElement51, FieldElement51)

Perform the Elligator2 mapping to a tuple (xn, xd, yn, yd) such that (xn / xd, yn / yd) is a point on curve25519.

See https://www.rfc-editor.org/rfc/rfc9380.html#name-elligator-2-method