Added equality function to Bag, and added unit tests

[nchlsb]

I saw that Bag did not have an equality function yet. I'm submitting an implementation that is imperative and optimized because I assume that is desired, but below is more declarative solution as well. Both pass the three unit tests included in this pull request.

equals(bag: Bag<T>): boolean {
        let allKeysSet = this.toSet();
        allKeysSet.union(bag.toSet());
        const allKeysArray = allKeysSet.toArray();
        return allKeysArray.every(element => this.count(element) === bag.count(element));
    }