Update to 1.6000
    - New upstream release 1.6000
      - Sync test files with Math-BigInt-1.999800
      - Update bundled Devel::CheckLib from v1.03 to v1.07
      - Math::BigInt::GMP is now a subclass of Math::BigInt::Lib, so remove pure
        Perl methods from Math::BigInt::GMP that are implemented in the superclass
        Math::BigInt::Lib; the methods removed are _digit(), _num(), _nok(), and
        _log_int() (the version of _log_int() implemented in Math::BigInt::GMP was
        buggy anyway)
      - Fix _check() so it doesn't give a "use of uninitialized value" warning if
        given an undefined "object"
      - Trim whitespace in all files
      - Better use of the functionality in Test::More in t/bigintg.t
    - Add support for build --with author_tests