Various fixes around ON CONFLICT for rule deparsing.

Neither the deparsing of the new alias for INSERT's target table, nor of
the inference clause was supported. Also fixup a typo in an error
message.

Add regression tests to test those code paths.

Author: Peter Geoghegan

src/backend/parser/parse_clause.c

@@ -2765,7 +2765,7 @@ transformOnConflictArbiter(ParseState *pstate,
 		ereport(ERROR,
 				(errcode(ERRCODE_SYNTAX_ERROR),
 				 errmsg("ON CONFLICT DO UPDATE requires inference specification or constraint name"),
-				 errhint("For example, ON CONFLICT ON CONFLICT (<column>)."),
+				 errhint("For example, ON CONFLICT (<column>)."),
 				 parser_errposition(pstate,
 									exprLocation((Node *) onConflictClause))));