add an option to dune build to report stats

[Khady]

Desired Behavior

It's fairly complicated to get an information about a build right now except for the total time taken. It would be nice to have an option to know more. Either on stdout or in _build/log could do. Maybe as part of --print-metrics. But having it on by default as much as possible and logging in _build/log would be nice.

Example

$ dune build
...
$ tail _build/log
known rules: 192789
executed rules: 16728
cache hits: 1281
number of source files: 200
number of compiled files 800
build duration: 17976ms

[rgrinberg]

I don't see any harm with this. In fact, we have something similar internally. Feel free to send a PR