OrderedIndex: support multiple-column ordered index

[Kybxd]

C++

To use struct as std::map keys in cpp, we should implement operator<:

struct Person {
    std::string name;
    int age;
    double height;

    bool operator<(const Person& other) const {
        if (name != other.name) return name < other.name;
        if (age != other.age) return age < other.age;
        return height < other.height;
    }
};

Go

To use struct as treemap keys in go, we should implement interface Ordered:

type Ordered[T any] interface {
	Less(another *T) bool
}

var _ Ordered[Person] = (*Person)(nil)

type Person struct {
	Name   string
	Age    int
	Height float64
}

func (p *Person) Less(another *Person) bool {
	if p.Name != another.Name {
		return p.Name < another.Name
	}
	if p.Age != another.Age {
		return p.Age < another.Age
	}
	return p.Height < another.Height
}

We should also create treemap.CustomKeyTreemap struct which accepts treemap.Ordered as key:

type CustomKeyTreemap[K Ordered[K], V any] struct {
    ...
}