Feature request: mark variables as closed over

It's possible to derive this from the scope tree, but slightly annoying. And it's useful for certain types of transformations: for example, assuming no dynamic scope,

```js
x && x.a();
x && x.b();
```
can be transformed into
```js
x && (x.a(), x.b());
```
if `x` is known to be local and `x` is known not to be closed over.