Programming a Guessing Game¹

To run the program:

$ cargo run --bin guessing-game
   Compiling guessing-game v0.1.0 ...

Lessons learned

Sometimes (extern crate rand) you just need stack overflow.
A String is both an immutable and mutable buffer of characters.
You create a new mutable variable with let mut:
```
let mut guess = String::new();
```
And read into it as a reference, i.e. &mut guess:
```
io::stdin().read_line(&mut guess);
```
Many Rust APIs return enums, such as Result. You can use .expect to panic:
```
result.expect("Failed to read line");
```

Or (pattern) match against the result, such as:

let guess:u32 = match guess.parse() {
  Ok(num) => num,
  Err(_) => {
    println!("Invalid!");
    continue;
  }
}