SourceCodeClassifier marks unused open declarations

![image](https://cloud.githubusercontent.com/assets/873919/3530402/235c77fe-07a4-11e4-92f4-95802ae5e057.png)

I will add tests but it basically works. 
I think it would be much better to make `FullNames` lazy. We're going to filter out a lot of symbols (those don't need open statements and those belonging to the same long idents), calculating FullNames upfront is wasteful.