Shifting Wares

Removed the map cache and its surrounding mechanics. Older maps can no longer be resold; they can 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.