Introduce convenience macros to hide JsonbContainer header accesses better.

This improves readability a bit and may make future improvements easier.

In passing, make sure that the JB_ROOT_IS_XXX macros deliver boolean (0/1)
results; the previous coding was a bug hazard, though no actual bugs are
known.

Nikita Glukhov, extended a bit by me

Discussion: https://postgr.es/m/9e21a39c-c1d7-b9b5-44a0-c5345a5029f6@postgrespro.ru