Instead of creating an iterator object at each step down the JSONB
object/array, we can just just examine its object/array flags, which is
faster.  Also, use the recently introduced JsonbValueAsText instead of
open-coding the same thing, for code simplicity.

Author: Nikita Glukhov
Discussion: https://postgr.es/m/7c417f90-f95f-247e-ba63-d95e39c0ad14@postgrespro.ru