Make as much of the interface as posible `constexpr`

[akrzemi1]

Make as many functions as possible in the interface of optional constexpr. It is not possible to be as constexpr as std::opitonal, because the latter is C++17, where constexpr is mor powerful, whereas boost::optional needs to be C++11-compatible.

This requires changing the implementation of optional storage to a union-based.

[akrzemi1]

For a union-based storage, see this implementation of Optional: https://github.com/akrzemi1/Optional

[typenametea]

Hello, I'll take a look into implementing this 🙂

[akrzemi1]

As indicated in https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3527.html#rationale.constexpr the constexprification can only work fro Ts that are literal types, and still, you cannot do it for the copy and move constructors in C++11. But you can constexprify them in later C++ verisons.

[akrzemi1]

Addressed in #143.