Run benchmarks from the CI

[g2p]

Results are expected to have lots of jitter due to virtualisation, but this is still convenient for catching large regressions.

[g2p]

@pascutto says mirage/index does continuous benchmarking;
https://github.com/mirage/index/blob/master/bench/bench.ml supports json output.
Not sure where the driver is though.

[pascutto]

There is no driver, it's a zero-config CI.
I don't know if it's ready to use in other repositories yet though.
/cc @gs0510

[g2p]

There was also #59 about sandmark