document HeapLang syntax and operational semantics
Progress towards #369 (the lifted WP rules are still missing).
This is what it looks like when rendered: iris.pdf. The new section is all the way at the end. We can iterate on this in-tree, but it'd be good if someone could at least read over it to make sure it is not total nonsense. :) I know some of the notation is a bit questionable, but unless you have concrete suggestions I propose we leave improving that to future work.