Function curve25519_dalek::montgomery::differential_add_and_double

fn differential_add_and_double(
    P: &mut ProjectivePoint, 
    Q: &mut ProjectivePoint, 
    affine_PmQ: &FieldElement51
)

Perform the double-and-add step of the Montgomery ladder.

Given projective points \( (U_P : W_P) = u(P) \), \( (U_Q : W_Q) = u(Q) \), and the affine difference \( u_{P-Q} = u(P-Q) \), set $$ (U_P : W_P) \gets u([2]P) $$ and $$ (U_Q : W_Q) \gets u(P + Q). $$