Generate timing reports with dune

[Alizter]

Dune doesn't know about the .timing files generated by Coq with -time.

We do however have a makefile, so I could imagine a sandboxed rule that copies all the .v files and runs make timings on them to generate the timing files and move them to a special directory target. It would be a bit hacky, but cleaner than running make yourself.

I'm creating this issue as a reminder for trying this.