Switch to using Set for Subscription teardowns

[benlesh]

Related to this issue here: #5638

1. Using an array of teardowns in our Subscriptions isn't ideal, performance-wise, for removing subscriptions.
2. Using a Set, rather than an Array would more closely mimic the behavior of most other common event registries, like EventTarget. Consider the following:

function handler() {
   console.log('handled');
}

const target = new EventTarget();
target.addEventListener('unsubscribe', handler);
target.addEventListener('unsubscribe', handler);
target.addEventListener('unsubscribe', handler);

target.dispatchEvent(new Event('unsubscribe')); // Only fires `handler` once!

target.removeEventListener('unsubscribe', handler);

target.dispatchEvent(new Event('unsubscribe')); // handler not fired (as it was removed)

This is in drastic contrast to our Subscription implementation, where the same subscription may be registered n times (which doesn't make sense, because it can only be torn down once), and the same function can also be registered n times, where it will execute n times.

Now, switching to using a Set internally would be a breaking change, as it would change how people can register and remove functions, more than anything. It's doubtful that it would be a breaking change for Subscriptions, because unsubscribe is idempotent.

Therefore we will not be able to do this until version 8.x.

[benlesh]

Core team meeting: Approved for next major.