Add additional tests to rotate functions, clarify variable names

[mdznr]

• Adds specific unit tests for the documented examples of the rotate functions. It’s good to specifically test the examples in the documentation. After all, those are the cases that are absolutely expected to work.
• Adds unit tests for _reverse(subrange:until:) and reverse(subrange:)
• Within documentation and implementation of _reverse(subrange:until:), use lower and upper instead of f and l. l could easily be misinterpreted as lower, left, or limit. Using lower and upper avoids this ambiguity.
• Use consistent variable names lower and upper instead of lo, and hi, respectively, in reverse(subrange:). This is more consistent with lowerBound and upperBound.

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

[mdznr]

Thanks for the review, @natecook1000. I’ve updated the code and I believe it reflects all the feedback given.

[natecook1000]

LGTM! Thanks @mdznr 🕺