Now that we have a go.mod, we can properly upgrade to blackfriday/v2
The output remains the same, but this PR brings back italic links with '_',
placing them into the link since this seems to be the more proper way.
Fixes#182
This bugfix implements correct way to calculate source sha1 hash, before it will changed,
by propagation of unchanged sources.
This commit will also include regenerated static files and *.hash files.
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.
