Add shared storage to property wrappers

[Syre]

@Syre

@propertyWrapper
struct Sharing<T, U> {
    
    var wrappedValue: T
    
    // @shared
    var sharedValue: U
}

struct Foo { }

struct Hud {
    @Sharing(sharedValue: 5) var foo = Foo()
}

// Every Hud's "foo" property shares the same value 5
let hud1 = Hud()
let hud2 = Hud()
let hud3 = Hud()

Whoops, this example has caused a bit of confusion, to be clear, I’m not proposing that sharedValue be a “special” property name akin to wrappedValue or projectedValue, it’s just the name I happened to pick for this “Sharing” wrapper. (Edit: maybe blessing sharedValue in this way is a good idea, but I don't feel strongly one way or the other on it.)
Additionally, sharedValue here was supposed to be a let, not a var.

In other words, this example is intended to follow exactly what is being proposed wrt shared storage in propertyWrappers, nothing more, nothing less.