jsonb array-style subscripting

Hi,

Some time ago the array-style subscripting for the jsonb data type was
discussed in this mailing list. I think it will be quite convenient to have
a such nice syntax to update jsonb objects, so I'm trying to implement
this. I created a patch, that allows doing something like this:

=# create TEMP TABLE test_jsonb_subscript (
id int,
test_json jsonb
);

=# insert into test_jsonb_subscript values
(1, '{}'),
(2, '{}');

=# update test_jsonb_subscript set test_json['a']['a1']['a2'] = 42;
=# select * from test_jsonb_subscript;
id | test_json
----+--------------------------
1 | {"a": {"a1": {"a2": 42}}}
2 | {"a": {"a1": {"a2": 42}}}
(2 rows)

=# select test_json['a']['a1'] from test_jsonb_subscript;
test_json
------------
{"a2": 42}
{"a2": 42}
(2 rows)

This patch has a status "work in progress" of course. Generally speaking,
this implementation extends the `ArrayRef` usage for the jsonb.
And I need some sort of advice about several questions:

* is it interesting for the community?
* is that a good idea to extend the `ArrayRef` for jsonb? If it's
appropriate, probably we can rename it to `ArrayJsonbRef` of something.
* what can be improved in the code at the top level (function placement,
probably, functionality duplication, etc.)?
* are there any special cases, that I should take care of in this
implementation?