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.
fanOut is only known when you have the whole dag, and you just have to look how many nodes point at a parent node. It can't change in the immutable dags, but for each modification of the graph, the ExpressionDag keeps track of the fanOut of each node.