Implement a faster `isInCheck`

[neongreen]

https://www.chessprogramming.org/Check

Currently a lot of time is spent on isInCheck

[neongreen]

NB: for simplicity we can just do the slow check if the last move is complicated (en passant or castling)

[neongreen]

The point is:

The cheapest and most straightforward way to detect checks is to consider the previous move just made by the opponent. It might be a direct check or discovered check, so one has to test whether piece on target square attacks the king, or whether piece on origin square is attacked by an own sliding piece on an otherwise open ray in opponent king direction [2]. If the last move was an en passant capture, one has to consider a few more complicated cases of possible discovered checks, since the captured pawn is not on the target square, and two pawns disappear from the 4th (5th) rank.