Computation of power polynomial is O(d * 2^d) instead of O(2^d)

[lucasxia01]

We compute the power polynomial with an outer loop that iterates over 2^d (d being the log size and also the number of betas), and use a O(d) inner loop that multiplies the appropriate betas together. We can optimize this to O(2^d) method that is also easily parallelizable by computing a binary tree.

Started in this commit.