Suggesting a chapter: compiler-macros

[commander-trashdin]

I'm thinking about a chapter dedicated to compiler macros. However, I'm not sure if this should be a chapter or an addition to an already existing chapter about macros. I guess (if we ever plan to have a reader-macros part as well) it makes sense to make them both parts of the macro chapter...? Anyway, I'll write a sketch (composed from several sources) and PR it. We'll see how big it gets. Of course if anybody has anything similar in mind, suggestions are welcome.

Something connected to it is a series of explanations about load-time values -- not sure where to put this one.