Debugflag to enable iterator stability error check

[martinus]

Add a define that enables error checking in robin_hood map, to check for invalid uses of iteratos after map/set is modified. This is mostly for debugging.

• Add a flag ROBIN_HOOD_ITERATOR_STABILITY_CHECK or something like that which enables/disables this code part:
  • Add counter member variable to robin_hood (initialized e.g. with a random number or just zero)
  • Counter is increased whenever the map is modified
  • All iterators get this counter when counter
  • accessing the iterator checks if the iterator's counter is the same as the map's counter, throws an exception otherwise

Thanks John for suggesting this!