CLN: Numba internal routines
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

- Remove some leftover implementation after https://github.com/pandas-dev/pandas/pull/36240
- Delegate numba func caching to methods in `numba_.py`
- Combine some helper methods
very nice!