Smarter items collector algorithm

[Windfisch]

This boils down to the Travelling Purchaser Problem. At least a very basic heuristic for this would still be better than the current algorithm, which just enumerates all chests around (0,0) or the player's position.

[Windfisch]

Opportunistic Collecting: Frequently-needed items should be collected if available, regardless whether they're needed right now or not