Add shares of curve points in circuit

[mzacho]

Main thing is changing Shares to

pub enum Shares {
    Nat(NatShares),
    Point(PointShares),
}

where NatShares is the old struct

pub struct NatShares {
    pub x: Nat,
    pub y: Nat,
    pub m: NonZero<Nat>,
}

and PointShares is the struct

pub struct PointShares {
    pub x: Point,
    pub y: Point,
}

[arcuo]

lit