`.map` with a callable returning a reference should return an optional reference

[TartanLlama]

Thanks to @ThePhD.

struct foo{
    int& v() { return i; }
    int i = 0;
};

int& x(int& i) { i = 42; return i;}

int main() {
    tl::optional<foo> f = foo{};
    auto v = f.map(&foo::v).map(x);
    static_assert(std::is_same_v<decltype(v), tl::optional<int&>>);
    //f.i should be 42
}