Add benchmark jobs

[Chris Warburton]

Laminar and ASV look like a good fit in terms of data model, etc. since
we can keep git clones in laminar's "workspace" directories, 'git pull'
them in the '.before' script (which we're already doing for builds), do
'asv run NEW' as our '.run' script and do 'asv publish', etc. in the
'.after' script.

Unfortunately there's a big problem: making sure that benchmarks aren't
run concurrently with any other job. Laminar doesn't support this
directly, although I've opened a github issue making a note of it
(purely for documentation purposes; not expecting it to be implemented).

The author suggested that as long as Laminar doesn't have this feature,
we could try altering the Laminar node configuration from within a job.
For example, we might have a 'foo-benchmark' job which does:

    sed -e 's/EXECUTORS=[0-9]+/EXECUTORS=1/g' \
        -i "$LAMINAR_HOME/cfg/nodes/desktop.conf"
    laminarc queue foo-benchmark-runner

Where 'foo-benchmark-runner' does the actual script running. I think
this would work for ratcheting down the executors, but it's less clear
how we'd increase them again. What if we just bump it back up in a
'foo-benchmark-runner.after' script?

I'll have to think about this a bit more...