Guide: guessing game

This doesn't rely on https://github.com/rust-lang/rust/pull/15690 for any code, but does for coherence.

We now build the game at the end of the first section.
    
I wanted to do it as we went along, but it's too hard with these fundamentals not in place. The rest will do the 'as we go' approach, but I think this is better.

Can’t this be:

```rust
match input.cmp(&secret_number) {
    ...
}
```

or is this intentional? Earlier it’s mentioned that you don’t need to write up your own functions a lot of the time because they’re already in the standard library. It seems a bit inconsistent to go on and define a function that’s already in the standard library.