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I was occasionally passing negative values to size_t.
Plus, this better reflects the intent of the variant argument.

Reported by Coverity:
https://scan8.coverity.com/reports.htm#v13253/p10402/fileInstanceId=22093891&defectInstanceId=7543346&mergedDefectId=174179&fileStart=251&fileEnd=500

Several string pointer arrays pointed to const strings
but were not const themselves.

A few various structures and arrays were also marked const.

(Also, '^' and '$')
Closes #508.

See #237.

Should fix #321.