Prophet4 rewrite status 3/24/20

Killer moves have been implemented. Two killer moves are kept per ply. A killer move is defined as a non-capturing move that causes a fail high. A new killer move is stored in “slot 1,” while the previous killer move in slot 1 is shifted to slot 2. Killer moves are tried after capturing moves, before any other non-capturing moves are even generated.

The next item on the agenda is to add draw checks into the search, then iterative deepening and “last PV” move ordering.