WIP: Fix let definitions eagerly included

This is an alternate candidate fix for #908.

* I still need to fix `SharedContext`, which I can do later.
* If we like this, I'll probably refactor things so that the `LetDefinitions` module is explicitly included into the example group just before it runs `before(:all)` hooks rather than using `before(:all)` directly.


@alindeman / @JonRowe -- I was having problems with `subject` when playing around with this last night (causing me to abandon the attempt at the time) but after sleeping on it, I found this solution.  Have you tried something like this?

Thinking about the solution of using an obfuscated name...I'm concerned about the fact that it means `super()` would now super to that name rather than the `let` name, which seems like it could break things in ways we haven't yet realized.

Thoughts?  Is this path worth continuing on?