remove unused OrderBy cases; clean up use of NonEmptyList
update test to accept result with temp field left over; tests on hand-written logical plans
propagate pipelines from sort keys
add optional hint to UnsupportedPlan error message
reinstate simpler handling of sorting on simple derefs

Restores simpler plan for simple cases, preserving source collection's shape.

A specific error, with an explanation of the cause
delete commented code