Look into restricting top-level verification of functions

[jennalwise]

We technically are not supporting the verification of functions, but right now Silicon is verifying functions when provided. This should not happen because the Silver parser should restrict this. However, ideally Silicon will warn against this and ignore verifying them in code. Alternatively, we can look to see whether we can support the verification similarly to methods. If so, make sure to not let functions be used in specifications!!