shiftRight

    pub fn shiftRight(r: *Mutable, a: Const, shift: usize) void {
        const full_limbs_shifted_out = shift / limb_bits;
        const remaining_bits_shifted_out = shift % limb_bits;
        if (a.limbs.len <= full_limbs_shifted_out) {
            // Shifting negative numbers converges to -1 instead of 0
            if (a.positive) {
                r.len = 1;
                r.positive = true;
                r.limbs[0] = 0;
            } else {
                r.len = 1;
                r.positive = false;
                r.limbs[0] = 1;
            }
            return;
        }
        const nonzero_negative_shiftout = if (a.positive) false else nonzero: {
            for (a.limbs[0..full_limbs_shifted_out]) |x| {
                if (x != 0)
                    break :nonzero true;
            }
            if (remaining_bits_shifted_out == 0)
                break :nonzero false;
            const not_covered: Log2Limb = @intCast(limb_bits - remaining_bits_shifted_out);
            break :nonzero a.limbs[full_limbs_shifted_out] << not_covered != 0;
        };

        const new_len = llshr(r.limbs, a.limbs, shift);

        r.len = new_len;
        r.positive = a.positive;
        if (nonzero_negative_shiftout) r.addScalar(r.toConst(), -1);
        r.normalize(r.len);
    }