Random function: Iterate over sequence of characters, not positions

[galtm]

Implementing an idea from @wendellpiez: Instead of iterating over a
numeric sequence that represents the position of each character in a
long string, iterate over the sequence of characters.

This change is in the implementation only, not in the function output.
I used ad hoc modifications of the label="seq-length=10000" scenario
in the XSpec test to check that the original and modified functions
produce the same UUID sequence.

Committer Notes

This PR simplifies some code inside a function.

This PR is not urgent, so no worries if the team can't review it for a while.

Acceleration wasn't an explicit requirement, but I used measure-time="true" in an ad hoc XSpec file running in Oxygen to compare the speed of the original and modified functions. The modified function here is slightly faster, e.g., 6% or 8% less time to generate 100,000 strings of length 100 each.

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This is not a core feature.

Changes to Core Features:

• [ ] Have you added an explanation of what your changes do and why you'd like us to include them?
• [ ] Have you written new tests for your core changes, as applicable?
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[wendellpiez]

Unit testing 🏅 great stuff @galtm

[david-waltermire]

Thanks @galtm !