(1) use a different algorithm that roughly halves the total number of
    multiplications required and results in more balanced multiplications
(2) use a lookup table for small arguments
(3) fast accumulation of products in C integer arithmetic rather than
    PyLong arithmetic when possible.

Typical speedup, from unscientific testing on a 64-bit laptop, is 4.5x
to 6.5x for arguments in the range 100 - 10000.

Patch by Daniel Stutzbach; extensive reviews by Alexander Belopolsky.