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