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.

- [ ] 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.
Yes. The reason was: 1) all the code just compiled with the change. 2) it made the graph uniform in that all AST nodes are either 0, 1, or 2-ary functions of `Producer[_]` for some value of `_`.