Guide: Variable bindings

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.
 1. You should use the term "String interpolation", not just "Interpolation", to distinguish this from e.g. linear interpolation.
  * The latter is what I get when I google for "interpolation" today; I had to add "string" to get usable links.
  * So, use of the full term will make it easier for the reader to get the right result when searching for more information.
   * (It would probably be a good exercise to go through and make sure every similar vocab word that is introduced or pseudo-defined in the guide currently gets a plausible result from google.)
 2. In particular, in this case I would say `"String interpolation is a programming term that means ..."
    * Further explanation for the latter suggestion: I read the use of "fancy" here as carrying an "aww shucks" tone, which I dislike, as it brings with it underlying theme of "don't worry your little head about big words like "Interpolation."  I see the latter as dismissive of the reader, rather than respectful.  But I suspect that others may think that an "aww shucks" tone it makes the text more approachable, so I will not push hard for this latter change.