Middle products and test code for arf, acf

[fredrik-johansson]

Timings for reciprocal of the power series 1 + log(1+x) to length n with 256-bit arf coefficients:

         n         old         new  speedup
         2    4.39e-08    4.46e-08  0.984x
         4    1.41e-07    1.39e-07  1.014x
         8    5.46e-07     5.4e-07  1.011x
        16    3.83e-06    2.98e-06  1.285x
        32    1.63e-05    1.23e-05  1.325x
        64    6.41e-05    5.63e-05  1.139x
       128    0.000174    0.000161  1.081x
       256    0.000422    0.000393  1.074x
       512     0.00102    0.000959  1.064x
      1024     0.00253     0.00236  1.072x
      2048     0.00662     0.00622  1.064x
      4096       0.019      0.0182  1.044x
      8192      0.0568      0.0524  1.084x
     16384       0.182       0.151  1.205x
     32768       0.641       0.497  1.290x
     65536       2.369       1.777  1.333x