Improve the experience of subscribing from within a listener

[lawrence-forooghian]

Currently you have to capture the return value of subscribe from within the listener, which Swift doesn't allow, forcing you to store it in a separate optional variable and then unwrap inside the callback. This is not great API. We should consider switching to something like the approach used by WIP ably-swift (a user-created signal that's passed in to the subscribe call; the signal then has an unsubscribe method).

┆Issue is synchronized with this Jira Task by Unito

[lawrence-forooghian]

Actually I just remembered we also pass the subscription to the callback, which is nasty in its own way; e.g. see #28