fix(reactivity): fix iOS 12 JSON.stringify error on reactive objects

- Use WeakMap for raw -> reactive/readonly storage. This is slightly
  more expensive than using a field on the taget object but avoids
  polluting the original.

- also fix Collection.forEach callback value

fix #1916

packages/reactivity/src/baseHandlers.ts

@@ -1,4 +1,12 @@
-import { reactive, readonly, toRaw, ReactiveFlags, Target } from './reactive'
+import {
+  reactive,
+  readonly,
+  toRaw,
+  ReactiveFlags,
+  Target,
+  readonlyMap,
+  reactiveMap
+} from './reactive'
 import { TrackOpTypes, TriggerOpTypes } from './operations'
 import { track, trigger, ITERATE_KEY } from './effect'
 import {
@@ -48,10 +56,7 @@ function createGetter(isReadonly = false, shallow = false) {
       return isReadonly
     } else if (
       key === ReactiveFlags.RAW &&
-      receiver ===
-        (isReadonly
-          ? target[ReactiveFlags.READONLY]
-          : target[ReactiveFlags.REACTIVE])
+      receiver === (isReadonly ? readonlyMap : reactiveMap).get(target)
     ) {
       return target
     }