Add Point method to check prime order subgroup inclusion

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.

No file or test is named. Start by locating the Point API and the subgroup order l, then review the proposed multiplication approach, addchain, and Pronin's technique in the linked paper. Done means the Point method's subgroup-inclusion behavior and implementation approach are agreed and covered by appropriate tests.

Written by the indexing model from the issue text.