* core/vm: fix incorrect computation of discount

During testing on Yolov1 we found that the way geth calculates the discount
is not in line with the specification. Basically what we did is calculate
128 * Bls12381GXMulGas * discount / 1000 whenever we received more than 128 pairs
of values. Correct would be to calculate k * Bls12381... for k > 128.

* core/vm: better logic for discount calculation

* core/vm: better calculation logic, added worstcase benchmarks

* core/vm: better benchmarking logic