Multiplicative and Additive Balancing

Given a tree of associative operations, say (((a * b) * c) * d), then rebalance the tree to be (a * b) * (c * d). This improves performance because the latter lowers the multiplicative depth by one. One can implement the algorithm provided in this paper:
[https://eprint.iacr.org/2021/1505.pdf](https://eprint.iacr.org/2021/1505.pdf).

This can also be useful for Additive Operations because it means that Additive Operations can be parallelized with better performance.

EDIT: This issue is addressed by #878, see below for more TODOs for a starter project.
See also #837 for a distributive property optimization.