scripts: Added codemapd3.py

Inspired heavily by d3 and brendangregg's flamegraphs, codemapd3.py is
intended to be a powerful high-level code exploring tool.

It's a visual tool, so probably best explained visually:

  $ CFLAGS='-DLFS_NO_LOG -DLFS_NO_ASSERT' make -j
  $ ./scripts/codemapd3.py \
          lfs.o lfs_util.o \
          lfs.ci lfs_util.ci \
          -otest.svg -W1500 -H700 --dark
  updated test.svg, code 35528 stack 2440 ctx 636

And open test.svg in a browser of your choice.

(TODO add a make rule for this)

---

Features include:

- Rendering of code cost in a treemap organized by subsystem (based on
  underscore-separated namespaces), making it relatively easy to see
  where the bulk of our code cost comes from.

- Rendering of the deepest stack/ctx cost as a set of tiles, making it
  relatively easy to see where the bulk of our stack cost comes from.

- Interactive (on mouseover) rendering of callgraph info, showing
  dependencies and relevant stack/ctx costs per-function.

  This currently includes 4 modes:

  1. mode-callgraph - This shows the full callgraph, including all
     children's children, which is effectively all dependencies of that
     function, i.e. the total code cost necessary for that _specific_
     function to work.

  2. mode-deepest - This shows the deepest/hot path of calls from that
     function, which is every child that contributes to the function's
     stack cost.

  3. mode-callees - This shows all functions the current function
     immediately calls.

  4. mode-callers - This shows all functions that call the current
     function.

  And yes, cycles are handled correctly: We show the deepest
  non-cyclical path, but display the measured stack usage as infinite.

For more details see ./scripts/codemapd3.py --help.

---

One particularly neat feature I'm happy about is -t/--tiny, which scales
the resulting image such that 1 pixel ~= 1 byte. This should be useful
for comparing littlefs to other filesystems in a way that is visually
interesting.

- d3 - https://d3js.org
- brendangregg's flamegraphs - https://github.com/brendangregg/FlameGraph

scripts/treemap.py

@@ -1141,22 +1141,24 @@ if __name__ == "__main__":
     parser.add_argument(
             '--to-scale',
             nargs='?',
-            type=float,
+            type=lambda x: (
+                (lambda a, b: a / b)(*(float(v) for v in x.split(':', 1)))
+                    if ':' in x else float(x)),
             const=1,
             help="Scale the resulting treemap such that 1 pixel ~= 1/scale "
                 "units. Defaults to scale=1. ")
     parser.add_argument(
             '-R', '--aspect-ratio',
-            type=lambda x: tuple(float(v) for v in x.split(':', 1)),
-            default=(1, 1),
+            type=lambda x: (
+                tuple(float(v) for v in x.split(':', 1))
+                    if ':' in x else (float(x), 1)),
             help="Aspect ratio to use with --to-scale. Defaults to 1:1.")
     parser.add_argument(
             '--title',
-            help="Add a title.")
+            help="Add a title. Accepts %% modifiers.")
     parser.add_argument(
             '--padding',
             type=float,
-            default=0,
             help="Padding to add to each level of the treemap. Defaults to 0.")
     parser.add_argument(
             '-l', '--label',