Provide analogue of `std::numbers` from C++20

[denzor200]

https://en.cppreference.com/w/cpp/header/numbers

Basically we will provide only inline constexpr double variables - like e, pi, log2e, ...
For C++14 we can additionally provide template<class T> inline constexpr T - like e_v, pi_v, log2e_v, ...