Revise chapter "Graphs and arena allocation"

[liamsi]

Hi, recently I started working my way through this chapter. It's the most interesting part, but it's quite difficult to read (at least compared to the other parts and especially if your are quite new to Rust, like me and I guess most people here).

For instance, I guess it would increase the readability to use less Rust code in the section-headings and also in the text. This means less Rc<RefCell<Node>> in the continuous text (only in examples or if the words fit into the text, i.e. 'Weak pointer' is fine). Another issue is the choice of words: It could be more consistent with the official Rust documentation (which c++ programmers will definitely consult, too). @nrc If you do not like these ideas, please let me know.

I would be happy to make some proposals and to contribute to this. Just started a branch some days ago (work in progress): https://github.com/Liamsi/r4cppp/tree/reading_graphs_arena I did not find the time to come up with good alternatives for every issue yet, but this issue is also a reminder to myself to do so.

Cheers and thanks again for this project :+1:

[nrc]

Hi @Liamsi, sorry it took a while to get back to you (been super busy). It is quite an advanced topic, much more so than the other chapters, so I think it will never be as easy to grok. Perhaps we should have a separate section in the contents for 'advanced topics' or something?

Anyway, I'd be happy for you to improve it! I'm not sure if there is a good way to describe Rc<RefCell<Node>> in a succinct way in english, please try though :-) Using more standard terminology sounds like a win.

I had a look through your branch and most of the changes look great! Please do send a PR when you're ready.

Thanks for the improvements!