CLN: drop internals._invert_reordering in favour of lib.get_reverse_indexer
I've implemented the former while refactoring BlockManager (#6745) because I didn't find the latter in lib module. This PR will remove the duplication (and also add missing changelog entry).

Speaking of missing changelog entry, I'm not sure about how do I mention an _internal_ class in Enhancements section...
 see the release notes / whatsnew for 0.13.0 when I refactored series. But don't think you need a whole section, just a line or two referring to the issue (and a warning/note to inform the user in v0.14.0 that hey did some refactoring internally, nothing in the API changed...bu wanted to let u know)
 looks fine, ping when ready
 Bumped the changelog. Is this what you had in mind?
 yep...that's good
 Travis failed with a weird error in io test, apparently, due to github serving `salary.table` with an extra empty line at the end.
 no prob...thanks
