Add missing operators of the form interval-plus-datetime, as required for

better SQL compliance in this area, per recent discussion.  Mark related
operators as commutators where possible.  (The system doesn't actually care
about commutator marking for operators not returning boolean, at the moment,
but this seems forward-thinking and besides it made it easier to verify
that we hadn't missed any.)
Also, remove interval-minus-time and interval-minus-timetz operators.
I'm not sure how these got in, but they are nonstandard and had very
obviously broken behavior.  (minus is not commutative in anyone's book.)
I doubt anyone had ever used 'em, because we'd surely have gotten a bug
report about it if so.