Relate exercises to relevant sections

[sellout]

Promoting one of the subpoints of #532 to its own issue: "It might also be nice to indicate somehow which exercises can be done after reading which sections of the chapter."

This seems like a very important feature to me. I've been reading this book in a study group and we only read a handful of sections between each session, so we have to try to identify (before having read the relevant sections) which of the exercises we should expect to complete by our next session.

It seems like if anything is done for the first edition, it would be to add some annotation or grouping to the exercises where they are. But I wonder if for the second edition it might be possible to integrate the exercises into the content of the chapter (or at least put them at the end of each section rather than the end of each chapter).