Reformat code in .pm files in bignum distribution.

Reformat code in .pm files in bignum distribution.

* Reformat code to make it more readable. The code should adhere to the
  guidelines in the 'perlstyle' manpage. All changes are in the amount
  of whitespace, e.g., all TAB characters are replaced by spaces. There
  are no changes in the way the code runs.

* Increment version number to 0.40.