Get rid of long-since-vestigial Iter node type, in favor of adding a

Get rid of long-since-vestigial Iter node type, in favor of adding a
returns-set boolean field in Func and Oper nodes.  This allows cleaner,
more reliable tests for expressions returning sets in the planner and
parser.  For example, a WHERE clause returning a set is now detected
and complained of in the parser, not only at runtime.