Fix for 5583. Handle expressions properly in DT[order(.)]. Tests added.
Fixes #5582. edge case for unique and duplicated, handles empty data.tables correctly.
closes #5519. dcast.data.table now runs fine on packages that imports data.table as well. NEWS updated.
Closes #5675 (column order when formula has … in RHS) ad #5676 (documentation for dcast).