Operations concerning dungeon level tiles.

Unlike for many other content types, there is no type Tile, of particular concrete tiles in the dungeon, corresponding to TileKind (the type of kinds of terrain tiles). This is because the tiles are too numerous and there's not enough storage space for a well-rounded Tile type, on one hand, and on the other hand, tiles are accessed too often in performance critical code to try to compress their representation and/or recompute them. Instead, of defining a Tile type, we express various properties of concrete tiles by arrays or sparse IntMaps, as appropriate.

Actors at normal speed (2 m/s) take one turn to move one tile (1 m by 1 m).