Iterator interface applied to generators has wrong type for current()

``` hack
function blarr(): Iterator {
yield break;
}

function main(): void {
$b = blarr();
$b->next();
$cur = $b->current();
// hh thinks $cur is int, but it's null
}
```

I'm not sure what the right solution is: making `Iterator::current()` return `?Tv` seems unfortunate when this isn't the case for most iterators.