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