Avoid unnecessary turns in path finder #196
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This introduces a new type HexCoord::Difference, which represents not absolute coordinates but differences between them (i.e. directions or steps). While both of them are quite similar, they are used differently in e.g. the arithmetic operations. Previously both were using the same type, but this change enforces using the right type in each place now.
When stepping paths, try to continue the last direction as long as possible while still retaining the optimal distance. This helps to avoid unnecessary turns (which then lead to unnecessary many waypoints in moves). Note that the implementation is very simple and just a greedy approach, so it is not (?) guaranteed to really minimise the number of turns. It does so quite well in practice, though. Since the path finder is not part of consensus, this change is not forking the game state (nor would further tweaks to the algorithm cause forks).
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This updates the logic in the path finder for stepping the path once the distance map has been computed. Instead of always choosing the next direction based on a fixed order of all six principal directions, we try to stick to the previous direction as long as possible. This is a greedy approach to minimise the number of turns taken, and thus the number of waypoints and sizes of moves. The resulting paths are still of optimal length.
This does not affect consensus (nor would future tuning of the algorithm), since path finding is no longer part of consensus since #145).