Allow elog_node_display() to show INSERTs

Hello,

I just made this patch so I could debug and otherwise
poke around.

It allows elog_node_display() and friends to show
INSERT parse trees.

AFIK it works, in my use and after applying
to the latest cvs with:
./configure --enable-cassert ; make check

The patch does not have a corresponding regression
test. I understand how to make a regular SQL-esque
regression test, but this is sorta a metadata kind
of test and I'm unclear on how to proceed or whether
there's even a testing framework for this sort of thing.

Karl <kop(at)meme(dot)com>
Free Software: "You don't pay back, you pay forward."
-- Robert A. Heinlein