Memory limit does not work as expected anymore

[sebschmi]

After changing to boxed types in the A* data structures, their size is not counted correctly anymore.

[sebschmi]

Fix is only temporary, need to decide if this or #91 is the better solution.

[jeeeesper]

After the discussion 2 weeks ago, will use this fix for now, as it covers our needs and is even a bit faster. So I'll close this issue to clean up a bit. Feel free to re-open when needed.