``long(str, base)`` is now up to 6x faster for non-power-of-2 bases.  The
largest speedup is for inputs with about 1000 decimal digits.  Conversion
from non-power-of-2 bases remains quadratic-time in the number of input
digits (it was and remains linear-time for bases 2, 4, 8, 16 and 32).

Speedups at various lengths for decimal inputs, comparing 2.4.3 with
current trunk.  Note that it's actually a bit slower for 1-digit strings:

  len  speedup
 ----  -------
   1     -4.5%
   2      4.6%
   3      8.3%
   4     12.7%
   5     16.9%
   6     28.6%
   7     35.5%
   8     44.3%
   9     46.6%
  10     55.3%
  11     65.7%
  12     77.7%
  13     73.4%
  14     75.3%
  15     85.2%
  16    103.0%
  17     95.1%
  18    112.8%
  19    117.9%
  20    128.3%
  30    174.5%
  40    209.3%
  50    236.3%
  60    254.3%
  70    262.9%
  80    295.8%
  90    297.3%
 100    324.5%
 200    374.6%
 300    403.1%
 400    391.1%
 500    388.7%
 600    440.6%
 700    468.7%
 800    498.0%
 900    507.2%
1000    501.2%
2000    450.2%
3000    463.2%
4000    452.5%
5000    440.6%
6000    439.6%
7000    424.8%
8000    418.1%
9000    417.7%