Add threading warning to Store.send (#772)

* Add threading warning to Store.send

Any async Effect (including fireAndForget) could result in a runtime crash when updating internal state like `effectCancellables`
This adds a debug warning to let users know

* Remove comment

Author: mluisbrown

Sources/ComposableArchitecture/Store.swift

@@ -353,9 +353,9 @@ public final class Store<State, Action> {
         )
         localStore.parentDisposable = self.producer.startWithValues {
           [weak localStore] state in
-          guard let localStore = localStore else { return }
+            guard let localStore = localStore else { return }
           localStore.state = extractLocalState(state) ?? localStore.state
-        }
+          }
         return localStore
       }
   }
@@ -390,11 +390,42 @@ public final class Store<State, Action> {
       var didComplete = false
       let uuid = UUID()
 
+      #if DEBUG
+      let initalThread = Thread.current
+      initalThread.threadDictionary[uuid] = true
+      #endif
+
       let observer = Signal<Action, Never>.Observer(
         value: { [weak self] action in
           self?.send(action)
         },
         completed: { [weak self] in
+          #if DEBUG
+          if Thread.current.threadDictionary[uuid] == nil {
+            breakpoint(
+            """
+            ---
+            Warning: Store.send
+
+            The Store class is not thread-safe, and so all interactions with an instance of Store
+            (including all of its scopes and derived ViewStores) must be done on the same thread.
+
+            \(debugCaseOutput(action)) has produced an Effect that was completed on a different thread \
+            from the one it was executed on.
+              
+            Starting thread: \(initalThread)
+            Final thread: \(Thread.current)
+              
+            Possible fixes for this are:
+
+            * Add a .receive(on:) to the Effect to ensure it completes on this Stores correct thread.
+            """
+            )
+          }
+
+          Thread.current.threadDictionary[uuid] = nil
+          #endif
+
           didComplete = true
           self?.effectDisposables.removeValue(forKey: uuid)?.dispose()
         },