Leorik 2.1

This version adds a pawn structure term to the evaluation: A dynamic bonus is awarded to passed pawns based on the phase of the game and their position on the board. Pawns being connected with or protected by other friendly pawns are also awarded a constant bonus. Isolated pawns receive a constant malus. The pawn structure term is only updated when a pawn moves or get's captured. A small and simple pawn hash table is used to avoid re-evaluating previously encountered pawn structures. With as little as 4999 slots a hit-rate of >90% is achieved and so the overall performance cost of adding this term is small. Leorik 2.1 is only a few percent slower than the previous version and gains about 50 Elo in strength.