Explicitly define objectWillChange on ViewStore. (#353)

* Explicitly define `objectWillChange` on `ViewStore`.

Because the `ViewStore` no longer uses `@Published` variables, the
conformance to `ObservableObject` no longer sythesizes the
`objectWillChange` property on `ViewStore` in iOS 13.0 or 13.1.

This caused `ViewStore` changes to not propagate, leading to views never
updating on these OS versions. This fixes issues #334 and (the already
closed) #238.

* Apply suggestions from code review

* Update Sources/ComposableArchitecture/ViewStore.swift

Co-authored-by: Stephen Celis <[email protected]>

Author: nsillik

Sources/ComposableArchitecture/ViewStore.swift

@@ -48,6 +48,10 @@ public final class ViewStore<State, Action>: ObservableObject {
 
   private var viewCancellable: AnyCancellable?
 
+  // N.B. `ViewStore` does not use a `@Published` property, so `objectWillChange`
+  // won't be synthesized automatically. To work around issues on iOS 13 we explicitly declare it.
+  public private(set) lazy var objectWillChange = ObservableObjectPublisher()
+
   /// Initializes a view store from a store.
   ///
   /// - Parameters: