Pure Perl module to test Math::BigInt with scalars

Changes for 1.999842 - 2023-09-27

• Fix a bug causing Math::BigInt and Math::BigFloat to be caught in an endless recursion. The bug was introduced when trying to fix https://github.com/pjacklam/p5-Math-BigInt/issues/8
• Allow div_scale to be 0. It is not documented what div_scale <= 0 means, but there are modules that set div_scale to 0 (e.g., Astro::Units) and fail their tests if this is not supported.