scripts: Added treemap.py

Based on the d3 javascript library (https://d3js.org), treemap.py
renders heirarchical data as ascii art:

  $ ./scripts/treemap.py lfs.code.csv \
          -bfunction -fsize --chars=asdf -W60 -H8
  total 65454, avg 369 +-366.8σ, min 3, max 4990
  aaaassssddddddaaaadddddssddfffaaadfffaassaassfasssdfdfsddfad
  aaaassssddddddaaaadddddssddfffaaadfffaassdfaafasssdfdfsddfsf
  aaaassssddddddaaaafffffssddfffsssdaaaddffdfaadfaaasdfafaasfa
  aaaassssddddddaaaafffffaaaddddsssaassddffdfaaffssfssfsfadffa
  aaaassssffffffssssfffffaaaddddsssaassssffddffffssfdffsadfsad
  aaaassssffffffssssaaaaasssffffddfaassssaaassdaaddadffsadadad
  aaaassssffffffssssaaaaasssffffddfddffddssassdfassadffsadaffa
  aaaassssffffffssssaaaaasssffffddfddffddssassdfaddsdadasfsada

(Normally this is also colored, but you know.)

I've been playing around with d3 to try to better visualize code costs
in littlefs, and it's been quite neat. I figured it would be useful to
directly integrate a similar treemap renderer into our result scripts.

That being said, this ascii rendering is probably too difficult to parse
for any non-trivial data. I'm also working on an svg-based renderer, so
treemap.py is really just for in-terminal previews and an exercise to
understand the underlying algorithms, similar to plot.py/plotmpl.py.

scripts/plot.py

@@ -554,6 +554,8 @@ def fold(results, by=None, x=None, y=None, defines=[], labels=None):
                     else:
                         y__ = None
 
+                    # do _not_ sum ys here, it's tempting but risks
+                    # incorrect and misleading results
                     dataset.append((x__, y__))
 
                 # hide x/y if there is only one field