Module 4 (datastructures) it too front-loaded on information

The module on datastructures suffers from a few issues:
- All the information is at the beginning, with exercises at the end
- The module is "hard" for day 1
- The module is often taught at the end of day 1, with concetration waning

These issues make the information bit too long and hard to follow. I propose exercises between the different sections. Logical points:

- After basic lists and indexing
- After slicing, `in`, `len`
- After tuples

找到 Module 4 datastructures 材料，并检查其中现有的章节和练习。在基本列表和索引之后、slicing、`in` 和 `len` 之后，以及元组之后插入练习；当信息在这些教学节点处分开时，即表示完成。

由索引模型根据 Issue 内容生成。