Improve Structs and Traits

[jotabulacios]

Review IsEllipticCurve, shortWeierstrassProjectivePoint, ProjectivePoint.

For example:

pub struct ProjectivePoint<E: IsEllipticCurve> {
    pub value: [FieldElement<E::BaseField>; 3],
}

or

pub struct ShortWeierstrassProjectivePoint<E: IsEllipticCurve>(ProjectivePoint<E>);

It could be defined with a named field like point: ProjectivePoint<E>, so we can avoid using .0.