Avoid deeply nested LinearCombinations in EvaluationsVar::interpolate_and_evaluate

[winderica]

Description

It is observed in privacy-scaling-explorations/sonobe#80 that stack overflow occurs when calling EvaluationsVar::interpolate_and_evaluate for large evaluation domains (e.g., when $n > 11$).

I think this happens because in EvaluationsVar::lagrange_interpolate_with_constant_offset and EvaluationsVar::lagrange_interpolate_with_non_constant_offset, we are updating the final evaluation result in a for loop, and += in each iteration wraps the current result by a new layer of LinearCombination. This produces a deeply nested LinearCombination. When we get .value() of the evaluation result, internally ConstraintSystem::assigned_value is invoked to recursively flatten the LC. Unfortunately, the number of LC layers of the evaluation result is so large that the recursion depth exceeds the OS limit, resulting in a stack overflow.

This PR fixes the above bug by calling .sum() on the iterator of addends, since this method invokes ‎AllocatedFp::add_many, which produces a flat LC.

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