This patch makes some minor improvements to the TODO list:
* add URLs for some TODO items (I think we ought to try to associate
at least one URL with each non-trivial TODO item)
* re-add a TODO item for an "estimated count"-like facility
* add a TODO item for considering changing how equality comparisons
treat NaN values, per recent discussion
Barring any objections, I'll apply this to HEAD tomorrow.
-Neil
