Implement O(sqrt(n)) `conditionally_select_power_of_two_vector` method

[mmagician]

Summary

Currently the function conditionally_select_power_of_two_vector uses method 5.1 from https://github.com/mir-protocol/r1cs-workshop/blob/master/workshop.pdf. By implementing the SumOfConditions method and then combining it with the existing technique, we could achieve less constraints.

For Admin Use

• [x] Not duplicate issue
• [x] Appropriate labels applied
• [x] Appropriate contributors tagged
• [x] Contributor assigned/self-assigned