With this change array fields are populated from json(b) arrays, and
composite fields are populated from json(b) objects.

Along the way, some significant code refactoring is done to remove
redundancy in the way to populate_record[_set] and to_record[_set]
functions operate, and some significant efficiency gains are made by
caching tuple descriptors.

Nikita Glukhov, edited some by me.

Reviewed by Aleksander Alekseev and Tom Lane.