Simplify CostMatrix interfaces

[shanemadden]

• Remove SparseCostMatrix, to be added back in screeps-game-utils crate
• Removed merge functions on LocalCostMatrix
• Removed set_multi function on CostMatrixSet, and replace set function with set_xy (helps differentiate trait function from struct functions, takes RoomXY); added CostMatrixGet with associated getter, implemented for both CostMatrix and LocalCostMatrix
• HasLocalPosition.. I don't think it ever was hooked to anything? Nuking it, as Position is the only thing that'd realistically ever be able to fit it (or maybe RoomXY, debatably - not worrying about it)