amcheck: Add gin_index_check() to verify GIN index

amcheck: Add gin_index_check() to verify GIN index

Adds a new function, validating two kinds of invariants on a GIN index:

- parent-child consistency: Paths in a GIN graph have to contain
  consistent keys. Tuples on parent pages consistently include tuples
  from child pages; parent tuples do not require any adjustments.

- balanced-tree / graph: Each internal page has at least one downlink,
  and can reference either only leaf pages or only internal pages.

The GIN verification is based on work by Grigory Kryachko, reworked by
Heikki Linnakangas and with various improvements by Andrey Borodin.
Investigation and fixes for multiple bugs by Kirill Reshke.

Author: Grigory Kryachko 
Author: Heikki Linnakangas 
Author: Andrey Borodin 
Reviewed-By: José Villanova 
Reviewed-By: Aleksander Alekseev 
Reviewed-By: Nikolay Samokhvalov 
Reviewed-By: Andres Freund 
Reviewed-By: Tomas Vondra 
Reviewed-By: Kirill Reshke 
Reviewed-By: Mark Dilger 
Reviewed-By: Peter Geoghegan 
Discussion: https://postgr.es/m/45AC9B0A-2B45-40EE-B08F-BDCF5739D1E1%40yandex-team.ru