Previously, only `Subscription` instances could be removed from a `Subscription`. Now any valid `TeardownLogic` that has been added to a `Subscription` may be removed by passing the same instance to `add`.

For example:

```ts
const teardown = () => console.log('called');

const subscription = new Subscription();

subscription.add(teardown);
subscription.remove(teardown);
subscription.unsubscribe();

// Will log nothing
```

Similarly with "unsubscribables":

```ts
const unsubscribable = {
    unsubscribe() {
        console.log('called');
    }
};

const subscription = new Subscription();

subscription.add(unsubscribable);
subscription.remove(unsubscribable);
subscription.unsubscribe();

// Will log nothing
```

- Cleans up and refactors Subscription substantially.
- Adds a few additional tests around Subscription and checking for teardown registries and unregistries

- Addresses comments
- Refines code a little further, moving some work closer to where it needs to be (teardowns array will no longer be allocated before we see if we even need to add anything, etc)
- Rewrites conditionals in _addParent method.
- Fixes test that didn't really test anything