Minor fixes for chapters 6, 7 and 8

[Vindaar]

Hey!

First of all thanks for the great resource! Really valuable and well explained.

I went through chapters 6, 7 and 8 over the last few days. I only realized halfway through 6 that the TeX source of the document is public. After I realized that, I fixed several minor issues when I encountered them.

Mostly it's typos and some (imo) stylistic improvements. In the Groth16 chapter I also added a few more equation references, because I found it very helpful to quickly glance back and forth and in some instances that was missing (evince's preview is useful for that!).

If desired I can add some screenshots for what the changed things look like (i.e. typesetting the Malbolge in \text{} for the 'Malbolge language').

Personally, I would probably also typeset other things (e.g. true / false) in \text when used inside math mode (the letter spacing is not ideal when just writing in math mode). If wanted, I can do that one of these days.

In one commit I forgot to disable auto-fill-mode in emacs, so the paragraphs are short there. I can fix that up (or line break everything to a sensible level, if that was desired).

Anyway, if you don't want to include one or more of the commits, just let me know and I'll kill them!