This is probably a bad idea because it will break every single use of Bisect_ppx.

However, I always found it annoying that the extension is .out.

• It conflicts with a.out when trying to run simple tests.
• It's easy to collide with output of other tools.
• It's not descriptive.

Any thoughts?

• Announce upcoming change to .coverage in 1.5.0.
• Make the change in 2.0.0.
• Show a message if the reporter receives a .out argument, or no file arguments.

There is no need to have a soft transition, because it's more effort than just making the transition.