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 could put a `Manifest[T]` there as well, but there are some legitimate cases where we don't know the T, but we have the K[_] and we just want the V[_]. Also, I'm quite afraid wiring manifests though will be painful (and I'm not sure we can do it).

Can we punt on this?