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.

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