Add debug shrinking log

[Gbury]

Should fix#71

cc @gasche , sorry for the delay.

[c-cube]

this is more a topic for discussion, not a review, but here Sys.getenv might also be quite convenient compared to passing parameters on the CLI (which needs to be negociated with each runner, including alcotest).
never mind, the whole PR is for the base runner.

[c-cube]

could this be a type counter = private {… to prevent mutation of the fields?

[Gbury]

That would be a good idea, yes.

[c-cube]

looks good, let's see if @gasche likes it :)

[jmid]

Should fix #71

First of all: Nice work! 👍🏻
I've recently spent some time debugging a non-trivial shrinker. For that I found logging each successful shrink attempt to a separate file (shrink01, shrink02, ...) and using the 2-column output of diff -y worked very well to visualize and understand the gradual shrinking process.

[gasche]

I just helped a student work on a shrinker for Inferno, and we benefited enormously of the new logging feature. Thanks!