Improve performance of Product2.distance(from:to:)

[timvermeulen]

This PR gets rid of any unnecessary computations in Product2.distance(from:to:). In particular, it now only does a single pass over base2 in the worst case (instead of 3), and for non-random-access collections it no longer does more index calculations than the default implementation of distance(from:to:).

Checklist

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

[natecook1000]

Looks great! And thanks for including the illustrative comments 🖼️