Add check extension to search

[vinc]

Check extension is quite simple, we already need is_in_check for null move pruning, so just before that we can add:

let is_in_check = self.is_check(side);

// Check extension
if is_in_check {
    depth += 1;
}

[vinc]

At the moment it seems to hurt the engine strength:

Score of Little Wing v0.3.0-52-g15293e6-ce vs Little Wing v0.3.0-50-g1ba57e7: 857 - 999 - 471  [0.469] 2327
Elo difference: -21.23 +/- 12.61
SPRT: llr -2.87, lbound -2.94, ubound 2.94
Finished match