gh-93065: Fix HAMT to iterate correctly over 7-level deep trees (GH-93066) (GH-93145)

Also while there, clarify a few things about why we reduce the hash to 32 bits.

Co-authored-by: Eli Libman <eli@hyro.ai>
Co-authored-by: Yury Selivanov <yury@edgedb.com>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>

(cherry picked from commit c1f5c903a7e4ed27190488f4e33b00d3c3d952e5)