Support producing multiple traces from runs

[shonfeder]

Closes #1360

This is a quick workaround to meet the need for this functionality, which has been voiced in the last few days.

quint run --out-itf=out-itf-example.itf.json --n-traces=5 ../examples/solidity/Coin/coin.qnt

Here's an example of how to use the new flag:

quint run --out-itf=out-itf.itf.json --n-traces=5 ../examples/solidity/Coin/coin.qnt

Then there should be 5 traces in a json array in the output file.

---

• [x] Tests added for any new code
• [ ] Documentation added for any new functionality
• [ ] Update other sub commands to also produce an array of itf traces, rather than a single trace
• [ ] Check with users to be sure this API is suitable to their needs
• [ ] Account for duplicate traces, and ensure traces are unique
• [ ] Entries added to the respective CHANGELOG.md for any new functionality
• [ ] Feature table on README.md updated for any listed functionality