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.
FWIW, while I do have some (decreasingly confused) thoughts on the Set[Id[_]], all I meant here is that in general, if you want to declare a type which ignores it's argument, you can equally do

```type IdSet[_] = Set[Id[_]]```

A wildcard on the left says "this takes a thing that we ignore," whereas a wildcard on the right is the more java-influenced wildcard.

More of a PSA.