Add in [Mate Distance Pruning](https://www.chessprogramming.org/Mate_Distance_Pruning) in the engine. See implementation in [hobbes](https://github.com/kelseyde/hobbes-chess-engine/blob/13f7e412977a80f1022d35cdf04df91b7d8f1c63/src/search.rs#L207-L213) and [carp](https://github.com/dede1751/carp/blob/7d5065bc26dd16002e6581b25abb3decf62ed86c/engine/src/search.rs#L148-L163).
