PERF: Use String HashTable in value_counts
- [ ] xref #14860 (Replace xxxx with the GitHub issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Roughly around 20-40% speedup in the benchmarkI added for largish 1000/10000 arrays. Benchmarks are pretty flaky, though.

duplicated/ismember didn't show improvement (actually regressed). I guess we aren't using the hashtable enough there to make up for the cost of doing ``is_string_array``.
This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. Closing to clear the queue, but feel free to reopen when you have time to revisit