STM clean-up

[jmid]

This PR cleans up the STM signature, leaving the repeat-retries combination property, which has worked well for Lin.

Currently, the numbers are set to repeat:25 and retries:15. We may want to adjust these before merging.

As a bonus

• it adds a negative int64 ref test
• removes a bit of whitespace

As an extra bonus, with the changes the Buffer test can segfault my local Linux running:

$ dune exec src/buffer/buffer_stm_test.exe -- -v -s 81407870

I'm therefore curious how the Linux CI reacts...

[jmid]

I should add that the Lazy tests take a while to run with these parameter numbers, which is why I am curious to reduce them...

[jmid]

OK, this actually segfaults with Linux CI as well:
https://github.com/jmid/multicoretests/runs/6372289400?check_suite_focus=true#step:6:1038

[jmid]

Rerunning jobs as trunk has now been patched for the MacOS segfault.

[jmid]

Given the MacOS CI red light, I've instead completed three successful runs of the test suite on the MacMini - and therefore declare this ready for merge! 😃