QL should give an error if a value is bound and never used

Summary: If I bind a value to a name and then don't do anything with it, that's almost by definition a bug. We should be detecting that and pointing it out somehow.

Really, this wants to be a warning. Ideally, we should detect it in the QL editor itself and flag it there, but not actually throw an error -- this construction is harmless in and of itself, but a very bad smell.