Use stack_data

[cknd]

Sooner or later this whole thing should be consolidated on @alexmojaki's stack_data, which is a cleaner, more robust and by now also more widely used version of the data extraction routines in here. I currently don't have time for a large refactor and all else things being equal would like to keep the formatted outputs looking more or less exactly like today, and this combination of constraints explains why this hasn't happened yet. See also https://github.com/cknd/stackprinter/issues/22 , https://github.com/cknd/stackprinter/pull/23 (This is not a new idea, just felt it should have an issue to refer to)