Warn for unused global 'let'.

Fixes the following:

```bash
cat > unused.js <<"END"
// jshint esnext: true, unused: true
let a = 42;
END
# should report unused 'a'.
./bin/jshint unused.js
```

Not sure how hacky it is to add an extra `unstack` but it should be the end of the parse by that point.