Core: Rename xi::optional to xi::Maybe, Add xi::Variant, xi::Flag

[zach2good]

I affirm:

• I understand that if I do not agree to the following points by completing the checkboxes my PR will be ignored.
• I understand I should leave resolving conversations to the LandSandBoat team so that reviewers won't miss what was said.
• I have read and understood the Contributing Guide and the Code of Conduct.
• I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

//
// Maybe<T>
//
// A wrapper around tl::optional to allow usage of:
// - Maybe<T&>
// - object.apply([](auto& obj) { ... })
//

//
// Variant<Ts...>
//
// A wrapper around std::variant to provide:
// - variant.has<T>() -> bool
// - variant.get<T>() -> Maybe<T&>
// - variant.visit(overload{ ... }) -> decltype(auto)
//
// https://en.cppreference.com/w/cpp/utility/variant
//

[WinterSolstice8]

Not sure what happened here at a glance but looks like this broke some tests somehow