Fix comparisons and behavior of is_sign_positive

[tspiteri]

This PR has three commits:

1. Small optimization on is_nan and is_infinite that saves one comparison in each.
2. Fix comparisons of +0 to -0 and comparisons involving negative numbers. Fixes comparison of +0 and -0 gives wrong result #2.
3. Update behavior of is_sign_positive and is_sign_negative to match the update behavior of the standard library. Fixes NaN behaviour of is_sign_positive and is_sign_negative is not like for primitive types #3.

[starkat99]

I'd been meaning to fix up some of this for a while now, so thanks a bunch for this!