This feature is born out of working our private fork of dune. Experience has shown that tracing is quite important and our so called "logging" is otherwise redundant. I therefore propose the following way to unify them:

1. Enable event tracing by default
2. Remove the log file
3. Add all log events to the trace
4. Introduce a dune trace --log command to filter out the so called log messages to reproduce (at least morally) what is currently our log file