Error if an evaluator column is unconstrained

The following code is accepted without errors or warnings:

```
ev is_binary([x, y]):
enf x^2 = x
```

I think this should be an error. It is probably best to reject any unused/unconstrained columns, because that is probably the uncommon case, and require an explicit annotation from the user for unconstrained columns (I suppose these would be used for nondeterministic inputs).

The goal of the above idea is to reduce the chance of a bug going undetected with a simple crude check.