Shifting Wares

Removed the map cache and its surrounding mechanics. Older maps will no longer be resold, in the sense that they will no longer be unlisted at all, until they meet the conditions to be fully discarded.

• Fixes #11
• Improves native compatibility with other mods. Custom implementations of map trades will no longer cause unsold maps to be discarded.
• Other mods can now more easily communicate data for other kinds of persistent items, without requiring a dependency on Shifting-Wares.

Upgrading a world from v2 will cause all cached maps to be discarded, even if they were never sold.