`bigfield`: Disallow (or handle) cases where some limbs are witness and some are constant in `bigfield::self_reduce()`

[suyash67]

An old TODO in self_reduce() function of the bigfield class suggests to handle the case when some limbs are constant and others are witnesses. I think we should not allow such a case (as the constructors were fixed to handle either all limbs being constant or witnesses). Investigate further.