Integers not correctly handled in `num::rational`

[tbu-]

• Possible-machine integers are treated as mathematical integers: Ratio::reduce, where the unary minus operator on a negative number is assumed to yield a positive number.
• self.denom is compared to Zero::zero instead of calling is_zero().
• Ratio::recip might return a fractions with a denominator of 0.

There are also some issues regarding overflows, it seems the module just hopes that they won't happen at all – except for wrong results, this could also raise a fail!()ure, when e.g. 1/sqrt(max_int) is multiplied with itself, yielding 1/0.

This might mean that Rust needs some general guideline on how to treat integer overflow.

[huonw]

#8466 is related.

[rust-highfive]

This issue has been moved to the RFCs repo: rust-lang/rfcs#337

[rust-highfive]

This issue has been moved to the RFCs repo: rust-num/num#11