Add enumeration method or implement Iterator in 2.x about di HOT 2 CLOSED

See #30

Instead of creating a specific purposed implementation, I've opted to create a generic method to retrieve all of the registered keys that can be used for specific purposes such as this.

from di.

The above implementation is a bit too naive because of the double iteration. If you iterate aliases and do something like buildObject() with them, it causes the contents of $resources to mutate, meaning you end up enumerating the same resources twice per iteration. This revised implementation addresses that problem by finalizing the list at the start of the iteration and ensuring there is no overlap.

    public function getIterator()
    {
        foreach (array_unique(array_merge($this->aliases, array_keys($this->resources))) as $key) {
            yield $key;
        }
    }

from di.