Implement Iterator & Countable interface for swoole_table

With this patch, we can iterate the table elements just like regular PHP array:

```php
$table = new swoole_table(1024);
$table->column('name',  swoole_table::TYPE_STRING, 64);
$table->create();
// add elements there

foreach($table as $key => $value) {
    var_dump($value); 
}

var_dump(count($table));  // get total elements num of the table
```

> NOTE
> due to the current limited underlying `swTable` implementation,  the real value of `$key`  doesn't stored in `swTableRow`(we only stored the crc32 hash value of the key), so we should not rely it in real apps, it may change without any notices in the future.