[WIP] @ResourceBacked property wrapper

[pcantrell]

A common idiom in Siesta projects is to have UI code point at some resource that can change, and whenever it changes, to:

• stop observing the old resource,
• add one or more observers to the new resource (the same observers every time), and
• load if needed.

The Siesta docs and the example project recommend doing this via didSet:

var doodadsResource: Resource? {
    didSet {
        oldValue?.removeObservers(ownedBy: self)

        doodadsResource?
            .addObserver(self)
            .addObserver(statusOverlay, owner: self)
            .loadIfNeeded()
    }
}

var doodads: [Doodad] {  // convenience accessor
    return doodadsResource?.typedContent() ?? []
}

Swift 5.2’s property wrappers give us a better option. This PR adds a @ResourceBacked property wrapper that lets any variable take its value from a Siesta resource.

The example above becomes:

@ResourceBacked(default: [])
var doodads: [Doodad]

// during initialization, e.g. in viewDidLoad() or some such:
$doodads.addObserver(self)
$doodads.addObserver(statusOverlay)

The doodads variable now behaves as if it is a read-only Doodad array; for example, we can do this:

for doodad in doodads {  // just works
    print(doodad.name)
}

However, that array comes from a Siesta resource — and we can switch which resource on the fly:

$doodads.resource = api.resource(“foo”).child(newDoodadID)

Changing the resource in this way automatically:

• removes our little flock of observers from the old resource (if any),
• adds our flock of observers to the new resource (if any),
• sends the appropriate events to the observers so they show the change immediately, and
• triggers a call to loadIfNeeded().

Future work on this feature

• The wrapper should detect whether the resource’s value does not match the variable’s type, and report this as an error via $doodads.error. This would give better error reporting to the user if the server returns well-formed but unexpected content. (If you ask for $doodads.resource?.latestError, you still get only underlying resource errors, since resources (at least in Siesta 1.0) have freely typed content.)
• Property wrapper should wrap response to observers to reflect the above
• Clients should be able to opt out of automatic loadIfNeeded
• Any facilities necessary for doing custom requests?
• Make sure it plays nice with SwiftUI