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

Locate the Module 4 datastructures material and review its existing sections and exercises. Insert exercises after basic lists and indexing, after slicing, `in`, and `len`, and after tuples; done means the information is broken up at these teaching points.

Written by the indexing model from the issue text.