Linter warns on unused for-of variable

The following is a pain to do in any other way when `iter` is an arbitrary iterable:

```js
let count = 0;
for (const unused of iter) count++;
```

But the linter warns that `unused` is never read. Possible workarounds would be (1) don't warn about unused variables whose names start with "unused", (2) possibly restrict this check to for-of variables.