Rework lesson on list, tuple, range; add enumerate & zip

[encukou]

Here's one more rework of existing lessons.

• A graphical overview of list mutation methods
• More thorough explanation of mutability
• Lesson on Nested lists, and enumerate & zip are split out
  • The enumerate & zip lesson has some more focus on how iterables behave in general
  • The enumerate & zip lesson includes unpacking in for
• New lesson on range, which should set the scene for enumerate & zip

Please review the changes as a whole, don't look at individual commits. But, to make cherry-picking easier, don't squash when merging.