`BigUint::from_str_radix` accepts strings with underscores as numbers

[01mf02]

I was very surprised to see that BigUint::from_str_radix("3_3", 10) yields 33. Given that I use BigInt inside my YAML parser, that made my YAML parser accept such numbers, even if they are not part of the YAML syntax. I definitely expected BigUint::from_str_radix to behave like i{N}::from_str_radix, especially given the identical name and input types. However, I did not see that this diverging behaviour is documented anywhere.

I would propose to either document this behaviour or --- what I would prefer --- to simply fail when a number contains an underscore. This would also make the parsing logic simpler. Anyone who wants to parse numbers containing underscores can simply strip them out manually before calling from_str_radix.

I would volunteer to make a PR to remove the underscore parsing logic.