Switch to something simpler for subscription storage

[lawrence-forooghian]

In 392fae3 I tried to be clever and treat the subscription storage as part of the mutable state (i.e. by making it a struct) but this got complicated because it needs to be able to update itself later (when the user unsubscribes), leading to a lot of unpleasant boilerplate. Turn it into a class instead (e.g. just copy EventEmitter from ably-swift).

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