More tests for sum types: records as option content

[ivan-gavran]

This PR adds to the sum type in the quint model one option which has a record as its content.
This will hopefully be useful as a reference because Quint cheatsheet takes this as an example for sum types.

[romac]

Great idea! I wonder if instead of adding a variant to Option we can instead define a new sum type and perhaps show two ways of having records with the variant in Rust, ie.

enum A {
  B(C),
  D { x: i32, y: bool }
}

where C is a struct, and with some better names.

What do you think?

[ivan-gavran]

Agree, it will make it clearer! I'll update the PR

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.08%. Comparing base (a141ced) to head (0090cb1).
⚠️ Report is 28 commits behind head on main.

Additional details and impacted files

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##             main      #14   +/-   ##
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  Coverage   51.08%   51.08%           
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  Files          14       14           
  Lines         693      693           
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  Hits          354      354           
  Misses        339      339           

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