Use variant for mson::Element

[tjanc]

Refactors mson::Element to use variant instead of a structure containing members for every type that could be stored in it. There are at least three more opportunities for improvement like this in MSON.h to be refactored in the future, matching this PR.

Principle

Let's say there is structure Content that can hold either Foo or Bar (but never both, i.e. a Σ-type). Currently, we use something like:

enum ContentKlass { UndefinedKlass, FooKlass, BarKlass };
struct Content {
  ContentKlass klass;
  Foo foo;
  Bar bar;
};

Much better, we can write:

class Content {
  std::variant<std::monostate, Foo, Bar> content;
public:
  const auto& foo() const noexcept { return std::get<Foo>(content); }
  const auto& bar() const noexcept { return std::get<Foo>(content); }
  template<typename F> void visit(F&& f) {
    std::visit(std::forward<F>(f), content);
  }
};

Benefits

• better approximation of our model (MSON)
• avoid initialization of members that are not used
• avoid manual switches and conditionals in code working with the MSON AST, and
• let the compiler ensure every code path on the structure is covered
• more functional code
• faster dispatch (variant produces a vtable)

[pksunkara]

Looks good to me.