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.
No, that would not be okay. Remember the comment above, if k1 == k2, then the types must be equal are you are going to have bugs.

We never violate that in the code we have here, but it is not a guarantee that the type system gives you. We have to guarantee that (which we do by keeping all modifications inside the ExpressionDag).