Re: jsonb status

Attached is v12. I think I've brought this as far as I can.

This is mostly just bug fixes, and some additional refactoring. I've
incorporated Andres' feedback. The only points that I think worth
noting are:

* The documentation has been significantly expanded to discuss
"containment" further, since it's a significant part of the feature.
This could probably use some polishing and general scrutiny, which is
something that Andrew may consider. I didn't have time to go over it
to the extent that I'd prefer.

* I altered containment semantics slightly. Now, it is not possible
for a "raw scalar" to contain a single-element array of the same
scalar, while the inverse is still possible (raw scalars still contain
themselves too). This made sense to me, and is consistent with the
behavior of the B-Tree operator class, where a raw scalar is not equal
to a single-element array of the same scalar. Rather, array is greater
than the scalar on the basis of its type alone, as at every other
nesting level. The fact that an array can contain a raw scalar is
explicitly presented as an exception to the general principle that
containment needs to be at the same nesting level.

I'm not going to go into the details of the bugs fixed, since no one
reported them here, and since they're trivial in nature. For full
details, you can review the log of our publicly accessible feature
branch.

--
Peter Geoghegan