fixed example/Makefile vs obsolete Python

  * add `VBox` and `HBox` to layout data blocks without the need to play
    with Heaps (fix examples accordingly)
+ * add frontend to generate Python stubs from C code
+ 

@@ -5,7 +5,7 @@ mp4: qs-partition.mp4 qs-main.mp4 stack-push.mp4
 %.pdf: %.py tpl.tex
 	ln -sf ../codanim .
 	rm -f out.*
-	python $<
+	python3 $<
 	pdflatex tpl
 	pdflatex tpl
 	cp tpl.pdf $@