Add Point method to check prime order subgroup inclusion

[FiloSottile]

Naively, we can do this by multiplying by l and checking if we get the infinity.

That multiplication can be precomputed as an addition chain generated with addchain.

Eventually, we should use Pronin's technique described in https://eprint.iacr.org/2022/1164.pdf.

[WeebDataHoarder]

I'm finding myself needing this to check whether the point is torsion free akin to Rust's curve25519-dalek is_torsion_free. Given that this library does not directly implement field.Element Sqrt for the paper implementation, I considered multiplying by the basepoint order (l) then checking against identity equality; however that's not directly allowed on current Scalar implementation (it must be strictly < than l).

Is this planned to get added at some point?