This will make future diffs easier to read because code changes modify the
hash/link, but the image data stays the same.
Note that there are whitespace diffs in the generated files where there
is no image. This is because there was a stray tab in the template before,
and now it's replaced by spaces.
Fixes#235
Consistently use `time.Sleep`, instead of `runtime.Gosched`, to ensure all
goroutines can make progress. `Gosched` wasn't working in the playground
(ref #149).
Also stop trying to compare operation rates. This was tenuous given e.g. how
short the programs ran for, and with the `Sleep`s we now expect the rates to
be similar anyways.
