Add shared storage to property wrappers

Evolution Pitches
40

Conceptually this should also work, subject to availability of Equality of functions. One of the workarounds discussed in that thread was to treat closure literal as a syntax sugar for compiler generated struct in certain contexts. With this workaround in mind, it could look like this:

struct Lazy<Value, Thunk: Function0, let thunk: @autoclosure Thunk> where Thunk.ResultType == Value {
    var _value: Value?
    var wrappedValue: Value {
        get {
            if let v = _value { return v }
            let v = thunk()
           _value = v
        }
       set {
           _value = newValue
       }
   }
}

func makeInitialValue(arg: Int) -> Bla { ... }

struct Usage {
    @Lazy<Bla, _, makeInitialValue(arg: 42)> var prop: Bla
}