`bigfield`: Simplify `bigfield::reduction_check()` method

[suyash67]

A TODO in the reduction_check() function of the bigfield class indicates unnecessary complexity in handling the circuit constant case. Investigate further to check if we can simplify this.

[suyash67]

We cannot use the assignment operator *this = reduced because reduction_check() is a const function. It prevents from using the assignment operator directly. What we do instead is to modify the mutable members of bigfield object (i.e., the limbs) to contain the reduced value.

Ideally, this function shouldn't be const because it is not just checking if reduction is required, it is actually performing the reduction as well (iff reduction is necessary). So we should split the function in two parts, one that checks if reduction is necessary and one that actually reduces. I think its not worth changing at this point as we'd have to change few lines of code in each bigfield function.