Schematics testing infrastructure: compare Trees, copy Trees

### Bug Report or Feature Request (mark with an `x`)
```
- [ ] bug report -> please search issues before submitting
- [x] feature request
```

### Desired functionality

When unit testing a schematic or a rule, I have sometimes found that it would be convenient if I could take two trees and compare them. Or at least ask whether the two trees are equivalent.

As part of this, it would also be convenient if there was some Tree-copying function - specifically for checking that a Tree has _not_ changed after some Rule call, but possibly for other cases as well.

首先檢查 Schematics 的測試基礎結構，以及 issue 中參照的 Tree 和 Rule API。確認樹的等價性和複製應如何運作，然後新增涵蓋範圍，證明等價的樹可以成功進行比較，並且可以在呼叫 rule 後檢查複製的樹是否有變更。

由索引模型根據 Issue 內容生成。