title: NSPoint as dictionary index
body: <p>I need to have data structure, each element in which is accessible by pair of (x,y) (like nspoint). </p>
  
  <p>I can store them in NSDictionary with keys like @"8,10", but I'm looking for more elegant and efficient solution (I don't want to have two dimensional array cause I want to have O(1) access to elements by coordinate)</p>