Reimplement level storage as a vector
	replacing previous implementation as a homebrew linked list.
	Levels are now stored in fixed-size block, LRU ordered. This
	changes the previous dungeon expiration behavior in that now
	the last 32 (or whatever configured) levels are saved rather
	than just city, country, and one dungeon. This simplifies the
	previously convoluted mechanisms for emerging from a house or
	from the sewers.
Overhauled the saved game format mostly due to the above, so saved
	games before this version are no longer loadable.