Utils cleanup

I had some free time and thought making some cleanup would help you contribute @marekpiechut.

Some changes to help pull in rgbkrk/atom-script#188.

@rgbkrk @marekpiechut Would you guys like to review this code?
Should we go one level further here and define a method such as `GrammarUtils.Lisp.statementsFromContext(context)`? If we do so we could remove the dependency on underscore form this file if we figure out how to remove the usage of `_.union`.