[ENH] introducing IntpHashMap and making unique_label_indices use intp
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them

Introduces IntpHashMap. Instead of making it from the scratch, we map the functionality either to Int32HashMap or Int64HashMap depending on the plattform. This has the advantage, that the resulting so/pyd-files don't get bigger.
