Feature/fix issue418

[JoshuaGreen]

This is my proposed fix to Issue #418. Analogously to Issues #413, #414, and #419, the "problem" seems to be that the move generation logic sometimes tries to move pieces off the board. As with PR #426, the simplest solution is to expand the array with appropriate slack space, a technique used elsewhere for the same purpose. In each of the cases here we (apparently) need only ensure that there's enough slack to enable stepping one space outside the board, and for move_diff_code this only requires adding one additional row.

As long as I was making the above change, I also took the liberty of abstracting the interface. No more remembering that we have to pass abs(from - to); instead we'll just call a simple function that will do whatever is necessary.