Add ScalaCheck shrinker to Nitpicker

[thpani]

To produce smaller counter-examples for easier human inspection:

• Shrink PBT-generated TlaType1 (witness type) in Nitpicker.
• Shrink TlaEx (witness expression) sets to sets with fewer elements.

Closes #1667, closes #1588.

[codecov-commenter]

Codecov Report

Merging #1683 (ac04bf2) into unstable (597f39f) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           unstable    #1683      +/-   ##
============================================
- Coverage     76.85%   76.84%   -0.01%     
============================================
  Files           383      383              
  Lines         11849    11849              
  Branches        537      537              
============================================
- Hits           9106     9105       -1     
- Misses         2743     2744       +1     

Impacted Files	Coverage Δ
...a/at/forsyte/apalache/tla/lir/TlaLevelFinder.scala	97.36% <0.00%> (-2.64%)	⬇️

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[shonfeder]

Looks like a great start! This'll be awesome. I've suggested a few small fixes and posed a few broader questions. I am maybe getting up to speed here too, so may just need some pointers on understanding, e.g., what's up with the expression shrinker.

[shonfeder]

LGTM!