Generator functions support

Added support for generators function, e.g.:
```
function fib() {
  var i = 0, j = 1;
  while (true) {
    yield i;
    [i, j] = [j, i + j];
  }
}

var g = fib();
for (let i = 0; i < 10; i++)
  print(g.next());
```

this commit is based on the destructuring assignment (PR #884) tree