MaxPrime

Moduli past this are refused. The splitting sweep runs over every element of the
field, so the cost is linear in the modulus where nothing else here is; at this
value together with MaxDegree the worst case is under 3*10^8 field
operations, which is a ceiling on a pathological input rather than a typical
cost. This is far below MaxPrime, which bounds
what the arithmetic can represent rather than what this can afford.