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.

(On 64-bit computers, the ratio is 184467440737095517:184467440737095516, meaning the small numbers are generated 0.00000000000000013553% more often than the larger one; i.e. it's definitely ok to not change this code. Feel free to just disregard entirely, but maybe this could be used to demonstrate importing traits to call methods later?)