Proposal: Add UniqMap

[nicklaus-dev]

Think about this scenario:

func main() {
	type User struct {
		Name string
		Age  int
	}

	// find unique names of users
	users := []User{{Name:"Alice",Age: 10},{Name:"Alice",Age: 12},{Name:"Bob",Age: 13}}
	names := lo.Map(users,func(u User, index int) string {
		return u.Name
	})
	names = lo.Uniq(names)
	fmt.Printf("%v\n", names) // [Alice Bob]
}

To find unique names, I need use Map and Uniq helpers. For this scenario, I suppose to add UniqMap helper.