Constant ring::arithmetic::bigint::modulus::MODULUS_MIN_LIMBS

pub const MODULUS_MIN_LIMBS: usize = 4;

The x86 implementation of bn_mul_mont, at least, requires at least 4 limbs. For a long time we have required 4 limbs for all targets, though this may be unnecessary. TODO: Replace this with n.len() < 256 / LIMB_BITS so that 32-bit and 64-bit platforms behave the same.