Fix `forEach` and `toArray` return types

[domfarolino]

Closes #27. Per discussion there, it seems we've landed on:

• toArray() causing a subscription, and asynchronously yielding an array via a Promise, which seems natural
• forEach() per https://github.com/domfarolino/observable/issues/29#issuecomment-1656292907, this also causes a subscription, so it shouldn't return an Observable. And since there's no accumulated or passed-through values, it returns undefined.