Add copyObject logic for TruncateStmt and a few other utility-statement

Add copyObject logic for TruncateStmt and a few other utility-statement
parse node types.  This allows these statements to be placed in a plpgsql
function.  Also, see to it that statement types not handled by the copy
logic will draw an appropriate elog(ERROR), instead of leaving a null
pointer that will cause coredump later on.  More utility statements could
be added if anyone felt like turning the crank.