Rewrite the treemap layout.

[mbostock]

• Improve performance and simplify code (particularly around rounding).
• Fix rounding errors (Treemap rounding is buggy? #2).
• Put the largest cell in the top-left by default (Treemap should put the largest cell in the top-left by default? #5).
• Don’t round by default (Treemap shouldn’t round by default? #11).
• Don’t populate area and z properties on nodes (Treemap shouldn’t populate area and z properties on nodes? #10).
• (Re-)Implement sticky layout.
• Redesign hierarchy.revalue (Remove hierarchy.revalue. #17).
• Return nodes sorted by depth (Sort nodes by depth then sibling order. #16).
• Implement sibling padding (Treemap should allow leaf padding. #1).
• (Re-)Implement asymmetric padding.
• Implement non-recursive layout (Treemap implementation should be non-recursive. #15).
• Rewrite so that the layout data wraps, rather than modifies, input data (Wrap rather than mutate input data? #13)?
• Rewrite to accept tabular, rather than hierarchical, input data (Accept tabular input data? #12)?
• Verify behavior when all siblings have zero or nan value (Treemap doesn't handle the level with all zero value correctly. d3#2419).
• Verify behavior when all nodes have zero or nan value (Treemap doesn't handle the level with all zero value correctly. d3#2419).
• Test everything, including old buggy rounding which I don’t yet have a test case for.