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.
We don't know the type for the next id to be allocated. Once we know the type, we allocate the Id[T](nextId). If we did it first, we would have to cast. The difference with roots is that in roots we never need the type, so again, it is type safe and there is no cast.