Implement LastModDown

[Janmajayamall]

This uses the same algorithm as ApproximateModDown (i.e. algorithm 2 of 2018/931) but scales by $q_l$ where $q_l$ is last prime in moduli chain of $Q$.

On input $x \in R_Q$, output $x' \in R_{Q'}$ where $Q'$ equals $Q$ after dropping last prime $q_l$ such that

$$x'i = [\lceil \frac{1}{q_l} x_i \rfloor]{Q'}$$

• Input can either be in Evaluation or Coefficient form. Try to accommodate it into a single function. If not, create two separate functions as LastModDownCoeff and LastModDownEval