Recognize unused 'let' at the global scope.
Simpler to do one 'unstack' here; "(blockscope)" already has one stack.
Always do the 'unstack', and after the last 'advance', for good measure.

This fixes the case where:

```js
// jshint esnext: true, unused: true
[a,b] = [1,2];
let c = 42;
```
Merge pull request #1053 from usrbincc/unused-global-let

Warn for unused global 'let'.