NotNaN binary operations can create NaNs

[ExpHP]

NotNaN's implementation prevents binary operators from receiving a NaN operand, but this is far from the only way that a NaN may arise;

NaN, the menace that it is, is something that all numeric binary operations are capable of producing!

fn main() {
    let zero = NotNaN::new(0.).unwrap();
    let infinity = NotNaN::new(1./0.).unwrap();
    println!("{:?}", infinity + (-infinity)); // ==> NotNaN(NaN)
    println!("{:?}", infinity - infinity);    // ==> NotNaN(NaN)
    println!("{:?}", zero * infinity);        // ==> NotNaN(NaN)
    println!("{:?}", zero / zero);            // ==> NotNaN(NaN)
    println!("{:?}", zero % zero);            // ==> NotNaN(NaN)
}

Notice that due to unsafe optimizations in NotNaN, this enables undefined behavior in safe code.

[ExpHP]

cc @ChristianBeilschmidt who implemented the ops

[reem]

Argh! I guess we should add checks for producing NaN and panic if we've done so.

EDIT: In places in the implementation where we now use NotNan() directly, we should use NotNaN::new with an expect which will add a check.