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.
Show me how to write that. I only see:
```scala
({type E1[t] = Expr[t, N]#E})#E1
```
which is clearly useless as I could have just written: `({type E[t] = Expr[t, N]})#E`. Maybe there is a cleaner way here, but your description is not clear enough for me to see what I could do.