Performance adjustments, more stats in performance report

I wasn't happy with the pause at the start of a walk with a lot of rooms (500+) for it to find and remove unreachable rooms, so I shifted it so that it handles them as it comes across them during the walk itself. This did result in one of the moves taking just over one second to calculate the next room, but I didn't notice it while it was happening, just discovered it in the stats.

Speaking of stats, dwalk report now also includes the longest move in the set, the longest time it took to calculate the next move, and the most checks done in any single calculation. Cuz... why not.

No real change otherwise.