Department of second thoughts: suppressing implicit casts everywhere in

Department of second thoughts: suppressing implicit casts everywhere in
ruleutils display is not such a great idea.  For arguments of functions
and operators I think we'd better keep the historical behavior of showing
such casts explicitly, to ensure that the function/operator is reparsed
the same way when the rule is reloaded.  This also makes the output of
EXPLAIN less obscurantist about exactly what's happening.