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.

Good call.