Fix a few issues with the IEEE-754 formalization.
- to_real x = 0 does not imply is_zero x, unless x is finite. - Add missing triggers. - Move any property related to signed zeros from "_finite" to "_special". - Fix incorrect signed zeros for addition, subtraction, and FMA. - Remove inconsistent signs of NaN for negation, multiplication, and division. - Add specification for special values of abs. - Fix useless specification for sqrt(+oo).