Macros: Split in topics, use playpen and cover more topics

I've rebased #185 and done a few changes. I'm satisfied with the current examples and the contents, but the wording could be further improved.

We are still missing the topic on how to export macros to be able to close #110 

@mdinger I hope you can review the changes
I think it's worth adding `(*identifier*)` here. Same with for `expr`, `pat` and `stmt`. Note: I actually have no idea what `stmt` is.