API: harmonize Index.to_flat_index to MultiIndex.to_flat_index
- [x] closes #23670
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

After "implementing" it (that is, just unifying the method), I'm even more convinced about my [idea](https://github.com/pandas-dev/pandas/issues/23670#issue-380340023)... but @WillAyd and @h-vetinari feel free to consider this just a stimulus to resurrect (and conclude) the discussion ;-)
