Re-arrange TOC.

I'm going to move testing to be right AFTER the guessing game. I wanted it
to be borderline TDD, but I think that, since the first example is just one
file, it might be a bit overkill.

I'm doing this in its own commit to hopefully avoid merge conflicts.
Guide: variable bindings.

Whew! Who knew there was so much to say about variables.

We probably want to move the guessing game to the rust-lang org, rather than
just having it on my GitHub. Or, I could put the code inline. I think it'd be
neat to have it as a project, so people can pull it down with Cargo. Until we
make that decision, I'll just leave this here.
auto merge of #15229 : steveklabnik/rust/if, r=pnkfelix

Whew. So much here! Feedback very welcome.

This is the first part where we actually start learning things. I'd like to think I struck a good balance of explaining enough details, without getting too bogged down, and without being confusing... but of course I'd think that. :wink: 

As I mention in the commit comment, We probably want to move the guessing game to the rust-lang org, rather than just having it on my GitHub. Or, I could put the code inline. I think it'd be neat to have it as a project, so people can pull it down with Cargo. Until we make that decision, I'll just leave this here.