Add check is_on_curve for jubjub points

[moCello]

Summary

We can check whether a point if the identity point and if it's part of the prime-order subgroup, but there is no check implemented that makes sure a point struct also verifies the curve equation, i.e. that the point actually is on the curve.

Possible solution design or implementation

Follow the implementation for the bls-points: https://docs.rs/dusk-bls12_381/latest/dusk_bls12_381/struct.G1Affine.html#method.is_on_curve

Additional context

This is needed to check for point validity in signature verification dusk-network/jubjub-schnorr#14