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