Re: Wanted: jsonb on-disk representation documentation

So, apart from cleaning up the code, we really need to take a close look
at the on-disk format now. The code can be cleaned up later, too, but
we're going to be stuck with the on-disk format forever, so it's
critical to get that right.

First, a few observations:

* JENTRY_ISFIRST is redundant. Whenever you deal with the Jentry struct,
you know from the context which element in the array it is.

* JENTRY_ISNEST is set but never used.

* JENTRY_ISBOOL is defined as (JENTRY_ISNUMERIC | JENTRY_ISNEST), which
seems confusing.

I'm going to proceed refactoring those things, which will change the
on-disk format. It's late in the release cycle - these things really
should've been cleaned up earlier - but it's important to get the
on-disk format right. Shout if you have any objections.

- Heikki