REF: handle dtype dispatch in libhashtable, share Vector/Hashtable code
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

can you rebase (havne't looked yet) ok looks fine, any perf diffs? (maybe expect small marginal improvements) > ok looks fine, any perf diffs? (maybe expect small marginal improvements)

i expect it to be a wash