[perl #83406] Library compatibility and API documentation (v1.991).

[perl #83406] Library compatibility and API documentation (v1.991).

- dist/Math-BigInt/lib/Math/BigFloat.pm: Increment version number.

- dist/Math-BigInt/lib/Math/BigInt.pm: Add workaround for library
  inconsistencies (Math::BigInt::Calc vs. Math::BigInt::GMP). This makes
  older versions of Math::BigInt::GMP work with latest version of
  Math::BigInt.

- dist/Math-BigInt/lib/Math/BigInt/Calc.pm: Correct and extend API
  documentation. Increment version number.

- dist/Math-BigInt/lib/Math/BigInt/CalcEmu.pm: Increment version number.