Change StringKey to simple strings. Fix ordering issue

[merriam]

Please review the following checklist.

• [X] Docstrings on all new or modified functions / classes
• [?] Updated documentation
• [n/a] Updated CHANGELOG.md (where appropriate)

DataTable's StringKey was recently exposed, meaning users could try to compare them. Unfortunately, this was causing an a comparison between str and None: StringKey("a string") < StringKey(). To solve the problem and simplify the class, either a StringKey value is the supplied string or the string representation of its id().

Test included, if you want to see it break before applying this change.