`std::nullopt_t` currently has no comparison operators. This prevents perfectly reasonable code from working, like `ranges::find(v, nullopt)` where `v` is a vector<optional<T>>, for no good reason.

Additionally, optional<T> has the full set of comparisons. But optional<T> is conceptually a variant<nullopt_t, T>, which wouldn't be comparable... because of `nullopt_t`. Other empty types like tuple<> and `monostate` are also comparable.

Proposed resolution: Add a defaulted member operator<=> to `nullopt_t`.