Surface persistence strategy of a Shared property

[stephencelis]

This branch introduces a second generic to Shared that tracks the underlying persistence strategy:

Shared<Value, Persistence>

This means that a particular persistence strategy can expose endpoints that the Shared could statically call. For example, a remotely-backed Shared could expose a refresh endpoint for fetching the latest data:

@Shared(.api(.config)) var config = Config()

// ...

$config.persistence.refresh()

Now this change comes with some complications and source incompatibilities with earlier periods in the beta. In particular, Shared properties that are not backed by a persistence key can no longer be specified simply:

@Shared var count: Int

Due to limitations in Swift (lack of default generics, for example), the new generic cannot be inferred in this situation. The type used to describe the persistence strategy of such a shared value is Any, and so there is a type alias to simplify:

// Instead of:
@Shared<Int, Any> var count: Int

// Do:
@AnyShared var count: Int

In the future, if Swift ever does get default generics, we could maybe get rid of these type aliases.

We'd love it if folks could take this change for a spin and/or provide feedback. We're trying to strike the right balance for shared state in terms of features and power without sacrificing too much in the way of usability and API surface area.

[stephencelis]

Going to close this for now. We can revisit in the future if we find more motivating examples, but for now it is possible to write a custom property wrapper that wraps a Shared and exposes endpoints, instead.