Performance tips: describe atsign-time and importance of allocation

This also moves the "Tools" section more up front. In general I tried to place things in the order in which users should explore their performance problems. (If you're worried about performance, it's silly not to mention the profiler until the end. I was the one who put it there originally, so it's my own silliness I'm complaining about.) Then the bulk of the document can be about programming practices.