PLonk/utils: opitmise multiply_all_polynomials
To multiply 4 polys p_1,.., p_4 with fft multiplication we compute fft_mul (fft_mul (p_1,p_2), fft_mul (p_3, p4). We propose here an optimization (and simplification) which computes fft_eval (p_1), ... fft_eval(p_4), multiply the results together and computes the interpolation.