Rusketh's benchmarking

A small, but very very useful, update to the E2 overlay by Rusketh. This
more accurately shows you how much server power an E2 is using up than
the ops. Ops costs of functions were mostly made up by the author of
that extension. But this actually shows the real cpu use. This update
makes E2 overlays display the time the server spent on each E2
execution.

The overlays used to say "Benchmark: Xs" but I changed it to "cpu time:
Xs" because I felt that better described what it does.