Issue #5512: speed up the long division algorithm for Python longs.

Issue #5512: speed up the long division algorithm for Python longs.
The basic algorithm remains the same; the most significant speedups
come from the following three changes:

  (1) normalize by shifting instead of multiplying and dividing
  (2) the old algorithm usually did an unnecessary extra iteration of
      the outer loop; remove this.  As a special case, this means that
      long divisions with a single-digit result run twice as fast as
      before.
  (3) make inner loop much tighter.

Various benchmarks show speedups of between 50% and 150% for long
integer divisions and modulo operations.