JSON serialization support would be nice

[judgej]

Was trying to convert a Sequence object to JSON, and just getting {} as the result. However, serializing $sequence->all() works just fine - gives me an array of items. Obviously the items themselves need to be serializable too, but that's not php-collection's problem.

So, could Sequence (and I guess the other collection types) be given JSON serialisation support? It is probably just as simple as adding this method and implementing \JsonSerializable:

    public function jsonSerialize() {
        return $this->all();
    }

I can provide a PR if this is something you would like.