Add implicit conversion from a wrapped value inside a variant

[acki-m]

Add support for automatic conversion of a wrapped value inside a variant:
Example use case:

int value = 23;
variant var = std::ref(value);
// instead of writing this:
std::cout << var.extract_wrapped_value().to_int() << std::endl;
// user can now write this:
std::cout << var.to_int() << std::endl;