Guessing game explanation

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.
auto merge of #15729 : steveklabnik/rust/guide_guessing_game, r=brson

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.