building timing reports with dune

[Alizter]

This PR adds support for building timing-html files as mentioned in #2230.

TODO

• Also put .timing files somewhere, for now only .timing.html is built
• Generate report of slowest lines
• Generate comparison report from last report

[jdchristensen]

This looks like a nice idea.

[Alizter]

I've added a way for dune to generate the .timing files. I'll need to now work out a way to compile this into a report about slowest lines.

[Alizter]

I've added a way of generating a report in _build/default/timing_report.txt. It uses the find script for combing through the .timing files. We can probably do something far more clever here.

The generated .timing files should also allow us to compare with a previous commit. I'm not sure what the best way to do this comparison is just yet. Hopefully I can draw some inspiration from the coq-tools timing scripts.