Point method to check for low-order points

For various applications where out of compatibility/specification reasons a switch to Ristretto is not feasible, it is important to check whether a point $P$ is of low order, i.e. $8P = O$.
Currently, this can be done manually using `MultByCofactor()` and `Equal()`. However, having a dedicated method for this could be more intuitive and efficient.
This is somewhat related to #33 and probably the most performant implementation follows [Pornin 2022, Section 3, Low-order points](https://eprint.iacr.org/2022/1164.pdf).

If such a method is relevant, I could prepare a PR.