Explain the use of Go's structs and composition for defining and using composite types and structures in Go programs?

In Go, a struct is a composite data type that groups together zero or more values of different types. It is similar to a class in other languages, but without inheritance or methods attached to it.

Structs are defined using the **type** keyword, followed by the name of the struct, and then the fields inside curly braces **{}**. Here's an example of a struct definition:

type Person struct {
    Name string
    Age  int
}

This defines a new struct type **Person**, which has two fields, **Name** of type **string**, and **Age** of type **int**.

Structs can also be composed by embedding one or more types within another type. This is similar to inheritance in object-oriented programming, but without the concept of subtyping. Here's an example:

type Employee struct {
    Person
    Salary float64
}

This defines a new struct type **Employee**, which embeds the **Person** struct as a field, and also has an additional field **Salary** of type **float64**. This allows an **Employee** to have all the fields and methods of a **Person**, as well as its own fields and methods.

Composition can also be used to implement interfaces in Go. By embedding a type that implements an interface, a struct can automatically implement that interface as well. Here's an example:

type Animal interface {
    Speak() string
}

type Dog struct {
    Name string
}

func (d Dog) Speak() string {
    return "Woof!"
}

type Bulldog struct {
    Dog
}

func main() {
    var b Bulldog
    fmt.Println(b.Speak()) // Output: Woof!
}

In this example, **Dog** is a struct that implements the **Animal** interface by defining a **Speak()** method that returns "Woof!". **Bulldog** is a new struct type that embeds **Dog**, and therefore also has a **Speak()** method that returns "Woof!". Because **Bulldog** has a **Speak()** method, it can be used as an **Animal** as well.

This is an example of how composition and interfaces can be used together to achieve polymorphism in Go.