The #overview command can provide some feedback about levels loaded
from bones files; that data wasn't being released at end of game.
(There are two copies of that data, one set always in memory with
the overview data [final_resting_place field in the 'mapseen' data],
and another set with portions attached to each relevant level [via
level.bonesinfo]. Neither set was being properly freed; this only
addresses one of them, so far. The per-level data can probably be
eliminated--for post-3.6--since DUNGEON_OVERVIEW isn't a conditional
feature as it was when that was implemented.)
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