Deprecate Streamable and StreamableUtil

[Sleitnick]

The Streamable and StreamableUtil modules were written to allow developers to watch for the existence of specific collections of instances in the Roblox hierarchy, as they stream in and out of existence on the client.

However, Roblox has finally added better control over how models are streamed in/out via the Model.ModelStreamingMode property and Model:AddPersistentPlayer() method.

With these new APIs, there is no longer a need for the Streamable/StreamableUtil modules. This is a very positive deprecation, as these modules should never have needed to exist.

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There are still cases when developers will want to watch for the existence of a collection streaming in and out. In these cases, it makes the most sense to tag the model/instance/etc. using the CollectionService.