Unit tests + cleanup of real.hpp

[PaulXiCao]

I worked on real.hpp. Nothing was fundamentally changed, more or less just formatting, refactoring, and extensive unit tests.

Important change: I removed constexpr from multiple overloads of <cmath> functions, e.g. abs, min, etc. See #275.

I went through all functions and tried to unify variable names, argument passing, etc.