Allow 1024 items per map tile, but limit to 1000 volume (500 for vehicles)

This does not affect mapgen, or map::add_item's default behavior. It's meant to make storage more realistic (volume limitation) and less of a hassle (since items don't stack), thus the player initiated inventory functions and actions will use this limit.

This brings the possibility of very large numbers, but I've attempted to ensure this isn't an issue; add_item_or_charges - when told to spillover - could potentially iterate through 25600 items (volume calculation) if all 25 surrounding tiles are completely filled, but it does so in around 10ms. Advanced inventory also handles 9216 items with no discernable slowdown. These numbers are likely to be atypical (volume limits would be reached for most things) would only appear if someone decided to spam a ton of paper bags or something.

Also, pickup handles amounts over 64 via extended identifiers (backtick + 2 letter combos), and of course arrow keys.