[feat] make serial_batch_inversion_and_mul public

[flyq]

I need to use batch_inversion_and_mul from ark-ff, and I hope to use both serial and parallel code.

The situation is like this, these use cases appear at the same time: some of them can be met by directly using the parallel feature, but others use case has additional complex logic. I parallelized the task in advance, so I expect the subtasks in it to be serial. And through measurement, if the parallel batch_inversion_and_mul is called directly in the subtask, it will lead to performance degradation, and it will cause additional mod div.

Adding this feature is simple. Change serial_batch_inversion_and_mul public:

pub fn serial_batch_inversion_and_mul<F: Field>(v: &mut [F], coeff: &F) {
...
}