Nested control flow

[moodymudskipper]

For #162

This works ok on given example and bquote but :

• tweak background color for a better look
• clean up flow_data, the logic should live in another function
• check how this affects flow_run etc
• the left right arrow doesn't always look nice/is not visible enough in png output, so we should explore emojis or alternatives
• I think we do need special casing for functions for the display to be nicer, use function() for anonymous functions.
• implement recursive boxes
• the way we implement data$nodes is not satisfying and will likely break the recursive boxes, should find a better way