Do not select nested group operations when the outer loop has a Sort

The alternative of preserving HKey operations across Sort_General, while somewhat more attractive, is impractical in the current system. There are two many row implementations with overlapping functionality. It might be simpler after a refactoring that made filling a row from storage (from Tuples or Protocol Buffers, say) more like building an array of values, such that only the table meta-data from the row-type was additional and not strongly bound to those values.