Add the operator Guess

[konnov]

Closes #888. This PR adds the operator Guess in Apalache.tla. The operator Guess(S) non-deterministically picks a value out of the set S. In this PR, Guess(S) and CHOOSE x \in S: TRUE behave the same. When #841 is closed, CHOOSE will become deterministic, whereas Guess will stay non-deterministic.

• Tests added for any new code
• Ran make fmt-fix (or had formatting run automatically on all files edited)
• Documentation added for any new functionality
• Entry added to UNRELEASED.md for any new functionality

[codecov-commenter]

Codecov Report

Merging #1590 (200e693) into unstable (625b215) will increase coverage by 0.01%.
The diff coverage is 85.00%.

@@             Coverage Diff              @@
##           unstable    #1590      +/-   ##
============================================
+ Coverage     74.99%   75.00%   +0.01%     
============================================
  Files           361      361              
  Lines         11753    11771      +18     
  Branches        571      576       +5     
============================================
+ Hits           8814     8829      +15     
- Misses         2939     2942       +3     

Impacted Files	Coverage Δ
...yte/apalache/tla/bmcmt/SymbStateRewriterImpl.scala	93.02% <ø> (ø)
.../apalache/tla/bmcmt/analyses/ExpansionMarker.scala	71.79% <0.00%> (-3.89%)	⬇️
...t/forsyte/apalache/tla/lir/oper/ApalacheOper.scala	66.66% <75.00%> (+0.87%)	⬆️
...e/apalache/tla/bmcmt/rules/ChooseOrGuessRule.scala	88.88% <88.88%> (ø)
.../at/forsyte/apalache/tla/imp/StandardLibrary.scala	100.00% <100.00%> (ø)
...forsyte/apalache/tla/typecheck/etc/ToEtcExpr.scala	94.47% <100.00%> (+0.04%)	⬆️
...in/scala/at/forsyte/apalache/tla/lir/Builder.scala	99.08% <100.00%> (+<0.01%)	⬆️
...ir/transformations/standard/KeraLanguagePred.scala	94.56% <100.00%> (+0.05%)	⬆️

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[thpani]

LGTM! Few minor comments below

[thpani]

I think we also need to update #888. It says:

We will do that and let the tool report that CHOOSE is not supported.

[Kukovec]

LGTM