Constexpr math (C++23) and arithmetic operators

[AbitTheGray]

Hi, I've read in #52 about aiming for C++11 support but did you consider supporting constexpr arithmetic operators and math in general?

I don't see any sideeffects fpm::ceil function so it should not be a problem when it is correctly version-checked.

Operators are a little trickier as you have the += implemented but marking it as constexpr won't work. That means static_assert(P{1} + P{2} == P{3}, "Arithmetics failed"); won't compile.

I expect inlining allows the compiler to process it at compile time anyone but it is not possible to use inside explicit constexpr functions.