Chapters 1-5, first pass

Great work Marijn! I've done a couple of passes on the chapters 1-5, and this is the first of them, including all the most important stuff. Separately, I'll send you another PR regarding what I saw as second-tier edits, more controversial than these were.

Do let me know if you have any feedback on the format or content of these edits, and whether they're in line with what you hope from me as a technical editor.

Looking forward to the objects chapter where all the crazy stuff comes in!