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

Sources/ComposableArchitecture/Store.swift

@@ -382,8 +382,40 @@ public final class Store<State, Action> {
 
       var didComplete = false
       let uuid = UUID()
+
+      #if DEBUG
+      let initalThread = Thread.current
+      initalThread.threadDictionary[uuid] = true
+      #endif
+
       let effectCancellable = effect.sink(
         receiveCompletion: { [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?.effectCancellables[uuid] = nil
         },