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.

Total nitpick: but this isn't perfectly evenly spread from 0 to 100: it generates 1-16 very *very* slightly more often than 17-100. A correct fix would be importing `Rng` and using `rand::task_rng().gen_range(1u, 101)`, which does some manipulations to ensure perfect uniformity (but I think this is too complicated for this point in the tutorial).