[range] contains method.
[btree_damage_visitor] Hide this behind a utility function btree_visit_values().
Remove level from btree_details::damage
[btree_damage_visitor] Introduce a DamagedKeys matcher in the tests.
[btree_damage_tests] << operator for thing.
[btree_damage_visitor_t] Store the btree path in the node_info rather than the level.
[btree_damage_visitor] pass the btree_path to both the damage and value visitors.