Inadequate infrastructure for NextValueExpr

Somebody decided they could add a new primnode type without bothering to
build out very much infrastructure for it.  Thus:

regression=# create table foo (f1 int, f2 int generated always as identity);
CREATE TABLE
regression=# insert into foo values(1);
INSERT 0 1
regression=# explain verbose insert into foo values(1);
ERROR:  unrecognized node type: 146

because ruleutils.c has never heard of NextValueExpr.  The lack of
outfuncs/readfuncs support for it is rather distressing as well.
That doesn't break parallel queries today, because (I think)
you can't get one of these nodes in a parallelizable query, but it
is going to cause problems for debugging.  It will also break
(more or less) pg_stat_statements.  I also wonder whether costsize.c
oughtn't be charging some estimated execution cost for it.

			regards, tom lane