Add support for ruint?

[javamonn]

Curious if you'd be open to a PR adding support for ruint in fraction and decimal as a num-bigint alternative. I think the constraints would all work. ruint supports conversions into num-bigint, but having native support would be nice.

[dnsl48]

hi @javamonn, sure, that would be a nice feature to have.
It also feels like a relatively easy thing to do. Both GenericFraction and GenericDecimal should work seamlessly with anything implementing the generic::GenericInteger trait.