Detecting if move that gives check exists

[R. Tomasi]

I finished implementing futility pruning. In the end I decided to go the route of using a custom movegen that only generates moves that give check. I'm more or less using a concept similar to what HGM describes on his website, with one addition: in positions where pawns are on rank7/rank2 (depening on the moving side), the futile gap is widened by the value of queen minus pawn, in the hopes of not making the engine blind to certain promotioncaptures. Not sure if that actually is worth any ELO, but it seemed like a reasonable thing to do, to me.