Give a semantic error on with statements

Pretty straightforward. Giving a semantic error on all 'with' statements, and skipping type check for them.
I was going to talk about how we should still typecheck the expression (since there's no `with` craziness there) and how the error message is inaccurate (since symbolless expressions like `3 > ''` ought to error anyway), but this is actually inline with the old compiler so I'm inclined to say it really doesn't matter.