Implement num traits for d128

[eugene-bulkin]

Would it be possible to implement the Float, Zero and One traits from the num crate? I know some other libraries (e.g. nalgebra) use these as trait requirements for doing calculations, so implementing them for d128 means we can use the struct in other crates that require such traits.

Namely, this requires implementing all the trig functions and some extra math ones. I can see integer_decode and classify being tough to implement, though.

[alkis]

It would but it seems like a lot of work. Is there a use case for it? Maybe you want to provide a PR?

[eugene-bulkin]

The main use case is usability with other crates, e.g. nalgebra. That crate implements matrices that work over anything that implements Float, Zero and One. Since it would be useful to do linear algebra on high-precision decimals, it would be nice.

As for providing a PR, I did plan on looking into it at least a little bit, to see how far I could get before I wasn't able to implement anything on my own. I think I'd mostly just be concerned with the trigonometric functions. I'll try something out in the near future.

[alkis]

Thanks Eugene!