Avoid overflow in cartesian_product size() impl

[tcbrindle]

Fixes #167

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6610792) 98.06% compared to head (b4c244d) 98.06%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #168   +/-   ##
=======================================
  Coverage   98.06%   98.06%           
=======================================
  Files          69       69           
  Lines        2424     2426    +2     
=======================================
+ Hits         2377     2379    +2     
  Misses         47       47           

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[tcbrindle]

Thanks for the review @isaacy2012 👍