Incorrect rounding for IBig/UBig to_f32/to_f64

[Shoeboxam]

I started fuzzing the subset of functionality that OpenDP depends on, and found the following bug:
https://github.com/opendp/opendp-num/tree/main/findings/dashu/DASHU-015

Overall map:

#89: RBig
#91: FBig/DBig
#93: UBig/IBig (this issue)

I can open a PR later today.

[cmpute]

Thanks for pointing out these. Will fix those and release a version by the end of the week

[cmpute]

Actually, I can let my AI agent pull all the findings in your repo and fix them at once