Adding fused div_add

[PaulXiCao]

Closes #146 .

This adds functions div_add and trait DivAdd. Tests are also added.

[redweasel]

Similar to the mul_add case you are testing against the "less precisely rounded version" and checking for equality. If those test succeed that means your implementation is just as precise as the default and you don't benefit from fma precision. Can you do a performance comparison to show in numbers, that fma is worth it? https://accurate-algorithms.readthedocs.io/en/latest/ch03fma.html suggests, that it has the same performance whilst theoretically achieving better precision if used right (not used right here).