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)
PERF: groupby / frame apply optimization
NETWORK: change failing FRED test
Merge pull request #7881 from jreback/apply_optimize

PERF: groupby / frame apply optimization
BUG/FIX: gut groupby.filter code, raise on multi-valued filter