treemap should be implemented with a better algorithm

[erickt]

This is a FIXME in the treemap code. Right now it's using a naive unbalanced tree, and should be reimplemented with a red-black tree or something else.

[graydon]

Left-leaning please: http://www.mew.org/~kazu/proj/red-black-tree/

[brson]

Fixed in #4487