copyObject() and equal() now know about all parse-time node types,

copyObject() and equal() now know about all parse-time node types,
including utility statements.  Still can't copy or compare executor
state, but at present that doesn't seem to be necessary.  This makes
it possible to execute most (all?) utility statements in plpgsql.
Had to change parsetree representation of CreateTrigStmt so that it
contained only legal Nodes, and not bare string constants.