Update to 1.40
    
    - New upstream release 1.40
      - Add changes for the newest release
      - Reorder change entries into descending chronological order
      - Update bundled Devel::CheckLib from v1.01 to v1.03
      - Update required version of Math-BigInt to 1.9994
      - Various test fixes to ensure correct backend is used
      - Rather than an early exit(), use skip() to skip tests