refactor(reactivity): use more efficient reactive checks

WeakSets and WeakMaps shows degrading performance as the amount of
observed objects increases. Using hidden keys result in better
performance especially when repeatedly creating large amounts of
reactive proxies.

This also makes it possible to more efficiently declare non-reactive
objects in userland.

packages/runtime-core/__tests__/misc.spec.ts

@@ -0,0 +1,18 @@
+import { render, h, nodeOps, reactive, isReactive } from '@vue/runtime-test'
+
+describe('misc', () => {
+  test('component public instance should not be observable', () => {
+    let instance: any
+    const Comp = {
+      render() {},
+      mounted() {
+        instance = this
+      }
+    }
+    render(h(Comp), nodeOps.createElement('div'))
+    expect(instance).toBeDefined()
+    const r = reactive(instance)
+    expect(r).toBe(instance)
+    expect(isReactive(r)).toBe(false)
+  })
+})