Incremental minmax

[oggy22]

Implement incremental minmax algorithm. For example if depth=10, the algorithm runs for depth=2 first, then 4, 6, 8 and finally 10. Each run will have killer moves initialized for the next run, thus it will speed up the search. Note that killer moves are stored per depth, so running the algorithm for depth=2 will initialize killer moves for depth=1,2, running it for depth=4 will initialize depth=1,2,3,4 etc.

This depends on #2