Implement underestimatedCount for Product2Sequence

[CTMacUser]

Add corresponding tests, one for an overflowed count, and one producing a proper count.

Saw this low-hanging fruit while planning other changes. No need for incomplete implementations if they can be realized within reasonable computational time.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary