fix(tiny-react): use symbol for node type

[hi-ogawa]

todo

• reproduction of { type: ... } collision
• serialization/deserialization doesn't need to be RNode/SNode
  • feat(tiny-react): generalize (de)serialize #244
• replace/revive symbol out-of-the-box?

---

Alternative would be to use __v_isVNode: true like vuejs?