A CompletedPoint is a point \(((X:Z), (Y:T))\) on the \(\mathbb
P^1 \times \mathbb P^1 \) model of the curve.
A point (x,y) in the affine model corresponds to \( ((x:1),(y:1))
\).
An EdwardsPoint represents a point on the Edwards form of Curve25519.
A FieldElement51 represents an element of the field
\( \mathbb Z / (2^{255} - 19)\).
A ProjectivePoint is a point \((X:Y:Z)\) on the \(\mathbb
P^2\) model of the curve.
A point \((x,y)\) in the affine model corresponds to
\((x:y:1)\).
The Scalar struct holds an integer \(s < 2^{255} \) which
represents an element of \(\mathbb Z / \ell\).