## stars

### Instructions

Write a function named `stars` that takes a number as
parameter and returns a string of stars (asterisks) 2^n long (2 to the nth power).

### Expected functions

```rust
pub fn stars(n: u32) -> String {

}
```

### Usage

Here is a program to test your function.

```rust
use stars::stars;

fn main() {
    println!("{}", stars(1));
    println!("{}", stars(4));
    println!("{}", stars(5));
}
```

And its output:

```console
student@ubuntu:~/stars/test$ cargo run
**
****************
********************************
student@ubuntu:~/stars/test$
```