Fix bug drawing embedded CDUs

If a CDU is contained in another, don't draw it until we've
already drawn its container
Experimental RST to SDRT converter
Try harder to separate STAC from generic glozz
Trivial RST discourse treebank id_to_path
RST: use space, not newline as inter-EDU separator
RST to SDRT: handle mononuclear/multi-sat
autodocs for rst graph/sdrt modules
A handful of RST test snippets
Tune gitignore to allow legit rst test snippets

Distinguish between artificial cases and those taken from corpus