Initial add of the generic optimizer code

Before digging into this code, look at the Rules implemented at the bottom of DagOptimizer.

There are several TODOS here:

- [x] After I split the code into separate files, I started hitting a scala 2.9 compiler bug, that I don't hit in 2.10. I'll try to work around, or possibly hope that Twitter finally gets on 2.9 before we need this.

- [x] Tests tests tests. I played around at the repl, but clearly we need super hardcore tests for any optimization framework.

- [ ] Actually call this code from an existing platform.
How about a comment?

:) This is `(HMap[E, N], N[T])` where `E[T] = Expr[T, N]` but there is that type lambda. I don't see a good way to remove the type lambda (that ({type ... }#E) because at the end of the day, N is not fixed and Expr takes two type parameters.