BUG: fix multi-column sort that includes Categoricals (GH7848)
CLN: refactor _lexsort_indexer to use Categoricals
BUG: Categorical concat should preserve levels (GH7864)
Merge pull request #7850 from jreback/cat_sort

BUG: fix multi-column sort that includes Categoricals / concat (GH7848/GH7864)