migrate libnum issues to the new repository

[thestinger]

https://github.com/rust-lang/num

• Add a BigDecimal type #8937
• ToBigInt and ToBigUint should not return an error #13555
• [UR] true "real" type, not a IEEE754 one. #11749
• Overflow on num::rational::Ratio #16782
• Integers not correctly handled in num::rational #16386
• Comparison of num::rational::Ratio is incorrect with overflow #8466
• Figure out a good convention for normalizing rational numbers #5738

[nrc]

Done. Although I'm not sure it was the right thing to do in some cases - implementation issues should stay here.

[nrc]

Whoops!

[nrc]

Ok, that seemed to have worked.